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Cam timing question with A65 750 #749352 09/16/18 8:23 am
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Mark Parker Online Content OP
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So we started up my son's 750 A65. It has new 38tm carbs with std jetting, 90 degree crank and std Spitfire/lightning cam and uses ign modules from a VT250 Honda, each cyl has it's own spark supply. I made a trigger system with trigger coils on an old points plate and very small diam rotor to trigger them.

In neutral it idles lovely but opening the throttle it goes Pa pa pa pa and will not respond to throttle nor pick up rpm, beyond perhaps 4000, no tach on it yet. It seems rich with a little fuel stand off with carbs opened a bit. Running it out of fuel doesn't improve it.

In theory the cam is timed the same as on my A65 and my brother's 750 90degree, both run nice. I use offset keys to retard the std timing a few degrees, and I'm thinking maybe my measurements were out and will check it and re-adjust if necessary, though it's idle is good?


mark
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Re: Cam timing question with A65 750 [Re: Mark Parker] #749353 09/16/18 8:47 am
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If it’s weakness, experimentally masking the intake with your hand will give it more fuel.

If your carburettors are too big, you need smaller ones (I guess!).


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Re: Cam timing question with A65 750 [Re: triton thrasher] #749358 09/16/18 11:02 am
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What on Earth is standard jetting on a 750 conversion with 38 mm carbs?


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Re: Cam timing question with A65 750 [Re: triton thrasher] #749360 09/16/18 11:17 am
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He means the TM38 Mikuni carbs are brand new and have the supplied standard jetting.

Re: Cam timing question with A65 750 [Re: Mark Parker] #749444 09/17/18 2:22 am
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We have used 38TMs on a 650 without problem plus I use them on my 883. I tried putting my hand over them to test if it was lean, plugs are covered in soot. I just re-checked the cam timing which is 31.5 - 58 inlet and 58-31 checking at 35thou clearance, so I doubt it is valve timing doing it. Lobe centres are 103.25 and 103.5 the 883 is 103-102 my brothers 750 103.5-102.5 his runs 38VMs and goes really nice. So I'll check what needle jets are actually in the carbs and what cut away, maybe they have different bits to what is specified, maybe the shop set them up for a 2stroke or something?


The other thing is maybe my ign triggers have a problem from having the two trigger coils in so close to each other.


mark
Re: Cam timing question with A65 750 [Re: Mark Parker] #749454 09/17/18 4:36 am
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Yea, that's the problem Nick, strobe works and doesn't, the R cyl is on 31.5degree approx, because it worked long enough to see it, L cyl I don't know because it flashed the strobe even less esp at any rpm. So haven't even sighted that timing mark yet frown

The trigger rotor is the inside of the std points cam, minus the cam with two bars opposite each other these are a bit over 4mm wide and not very high, it needs to think it's crank triggered so the advance works before 6000 otherwise it would only need the one bar, maybe the bars are too wide or something?


Jetting checks as std, they have no air jet.

Last edited by Mark Parker; 09/17/18 9:23 am.

mark
Re: Cam timing question with A65 750 [Re: Mark Parker] #749457 09/17/18 4:42 am
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Mark Parker Online Content OP
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Maybe I should get two of these? The Honda ign and coils don't give much spark.

https://www.aliexpress.com/item/Ign...-Scrambler-500-1999-ATV/32813453545.html


mark
Re: Cam timing question with A65 750 [Re: Mark Parker] #749464 09/17/18 6:13 am
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Hi Mark

While I don't know anything about setting up TM carbs. Do you happen to have a pair of smaller carbs, say 34mm ones that you can try! This should help the lower RPM pick up atomising. Plus as Nick mentioned you can increase the accelerator jet if needs be.
When the two strokes came out with the "power jet - read accelerator jet" system it was to get cleaner acceleration at lower RPM, with the power jet there to supply more fuel at larger throttle opening.

just throwing my tuppence worth in the mix

John

Re: Cam timing question with A65 750 [Re: Mark Parker] #749467 09/17/18 9:20 am
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Mark Parker Online Content OP
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I'm suspecting it's the ignition trigger system I made. The coils are correct for the units and they work very weakly. I'm wondering if the trigger system can effect the spark intensity?


I put red paint in the trigger rotor lug that fires on compression, and there is red paint on the L/h cyl trigger coil and its timing mark on the alternator so I don't mix the timing marks.


What is different on this bike is the small trigger rotor and the trigger coils being mounted on the points plate. Other ones I've done have trigger coils mounted much further out on the case with much longer trigger bars which would pass the coils at higher speed.

[Linked Image]
[Linked Image]

Last edited by Mark Parker; 09/17/18 9:25 am.

mark
Re: Cam timing question with A65 750 [Re: Mark Parker] #749735 09/19/18 10:46 am
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I'll try facing the pickup coils toward the base plate where the adv retard sat and fix two lugs where the pivot posts for bob weights used to be. That moves the trigger points out on a larger circle still on a movable points plate, and testing a bodged up version, with a screw head sitting up, it triggers nicely at a slower rotation speed.

Pulling the H/T leads out of the coils got the strobe to work with the set up in the photo above last night, but only at low rpm, after that the strobe stopped and the motor sort of cut and will not run right to pick up rpm, I doubt it's mixture. If I can mount the pickup coils on the points plate so triggers work good, even if it runs ok with the Honda ign units I might try a couple of the Chinese DC CDI units with their coils, and see what sort of spark they give by comparison.


mark
Re: Cam timing question with A65 750 [Re: NickL] #749742 09/19/18 12:48 pm
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Originally Posted by NickL
The CDI unit will give you a very intense spark with a short duration, the difference between the different types are typically
HEI style which you have at the moment approx 1-1.4 milliseconds, CDI typically 0.1- 0.2 milliseconds.
With a hemi type combustion chamber the short duration may give an incomplete burn with large valve overlap, sometimes
making the engine appear to have flat spots. I have built many versions of CDI for racing purposes and use a much more powerful
inverter and storage capacitor than those shown in the photo. If you use a CDI, transformer style coils improve matters considerably,
they have more closely coupled coils and so greater pri-sec inductance, they also have fewer secondary losses.
The Honda system is a better way to go for general purpose use.


Most all the CDI's sold for automotive use are multi spark at lower rpm's to make up for the short spark duration..I found that a short duration spark did make a flat spot on a modified vintage auto engine with large non hemi combustion chamber. The large carbs may go lean on throttle initial opening contributing to the flat spot...In my experience, some engines respond better with a a larger plug gap if the ignition has enough energy...And depending on fuel quality, more spark advance can help response especially if there's a brief lean out when the throttle is opened...but may be too advanced on top end unless the total advance is changed..
I find the remarks on stock Mukuni TM main jets interesting...The 34 TM's I use come stock with a 260 main jet that is three sizes too large for a modified 650 Triumph as tested on the dyno and the track...I would suspect the larger TM's follow the over rich stock jetting?


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
Re: Cam timing question with A65 750 [Re: Mark Parker] #749838 09/20/18 8:14 am
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Made a new trigger system, so much better, starts very easy but does the same thing. But I had a bit of a light's on moment. When I put it together one cyl had no spark so I swapped the bigger of the two ign boxes for another small one, same wires coming out of both. However in the big box there is a big component, a silver box with HI133 on it, the other boxes do not have this, why they are smaller. All the boxes have in common is power supply 12v and earth otherwise they are separate ignitions, now I'm thinking the HI133 might be meant to do something for both ignitions that's not getting done?


mark
Re: Cam timing question with A65 750 [Re: Mark Parker] #749841 09/20/18 9:01 am
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[Linked Image]

The trigger system consists of tapping some holes bigger in the plate where the auto adv sits and fitting two screws and locknuts to adj the height of the screw heads. The trigger coils are face down and they are about .5mm lower than the bottom surface of the points plate, so I can get a suitable gap between them and the screws triggering them, they fit nicely under the outer cover and I can adjust them a little individually to get each cyl spot on. Will wire the big box back in and see if it will work, hope that is the problem, idles great.

[Linked Image]


mark
Re: Cam timing question with A65 750 [Re: Mark Parker] #749846 09/20/18 10:11 am
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Wired the big box back in but it does not work, left cyl is fine. Almost everyone calls these ign boxes CDI, can find one of the two ign box's on eBay but it looks like another small one. Maybe 2 12V DC 'CDIs' from China might be the go. CDI may not necessarily be what they are, there are two types, 12v powered through the battery and AC powered.


mark
Re: Cam timing question with A65 750 [Re: Mark Parker] #749859 09/20/18 1:17 pm
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Ben ordered two units with coils from China they have advance curves, I'm not sure which version he got, other than DC 12 v. Probably not adjustable. He asked about them and they have around 24degrees of adv. which should be ok. No rev limiter smile My guess is the Honda system needs a big and a small box used together, that's how Honda used them, 12v goes to both boxes and both coils and maybe that HI 113 component has to be there, to do something, the small boxes do not have that. The ignition consists of a big and a small, I used two small boxes because the big one is kaput. I'll scotch lock the big box off my bike on in the morning and see if it will do more than being just off idle, it idles great just cannot turn the throttle without it cutting out. If the one off my bike makes it run properly we might be able to find another Honda module, the wrong one on eBay is $60 possibly more expensive than the coils and units from China that are coming.


mark
Re: Cam timing question with A65 750 [Re: Mark Parker] #749894 09/20/18 6:32 pm
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Did you means SV 650 Suzi unit Nick ? , its a V twin. I know nothing about these things, my Cagiva with an SV 650 motor has one pick up, signal goes to a black box ( EMU?) , I havent got any experience with what goes on where with these, it just works. The ign pick up is mounted by the alternator, its meant to be fixed so the timing is set , of course , this can be changed, I slotted mine and advanced it a wee bit to make it perkier.

Last edited by gavin eisler; 09/20/18 6:35 pm.

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Re: Cam timing question with A65 750 [Re: Mark Parker] #749909 09/20/18 10:20 pm
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My 96 Ducati 900 M has two pick ups on the flywheel under the side cover. It has two Kokusan Denki control boxes about 1/3 the size of a pack of cigarettes and two coils. So each cylinder has it's own ignition system and each is a waste spark system..The coils measure 3.5 ohms..Maybe Nick knows if it's a CDI type...It has no problem firing a .035 gap at 8500 rpm ...


79 T140D, 96 900M Ducati ,2001 Sportster....On a bike you can out run the demons..
Re: Cam timing question with A65 750 [Re: Mark Parker] #749941 09/21/18 3:41 am
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I put my ignition box on, it works but is not the problem, it's fine till around 1/8 throttle then 8 strokes with fuel stand off, with a bit of black smoke, if you open it it cuts it out. My bike runs fine, and the carbs have similar settings. I might try my carbs. Maybe it needs leaner needle jets these are midrange in what's available. Plugs get lots of soot.


I think they were answering on crankshaft degrees Nick. Probably the two small Honda box's may work fine if I can locate the actual problem, that doesn't seem to be ignition, Cam is retarded a bit, as on my bike and my brother's 750, pipes are as we used before.


mark
Re: Cam timing question with A65 750 [Re: Mark Parker] #749963 09/21/18 10:06 am
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Sorry if this has been mentioned, but the igntion isn't over advanced is it ?


beerchug
Re: Cam timing question with A65 750 [Re: Mark Parker] #749965 09/21/18 10:10 am
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It seems like reversion, the pipes are fairly big diameter and the inlet is closing later. As soon as you open the throttle fuel sprays out the carbs. I'll try putting some anti reversion plates into the pipes. This head flows around 190cfm and the ports, though smaller than on my 883 are big. This is probably hottest A65 I've built, at least in what hp we might get from the displacement. We could put std size headers on and increase gas speed it the exhaust and inlet, but that would hurt hp at higher revs.

The inlet needs more gas speed to eliminate reversion, if air fuel has insufficient speed/inertia when the piston starts to rise on the compression stroke it will blow the mixture back out the carbs before the valve closes. Maybe that's what it's doing.


This is the trigger mechanism, fully adjustable so set clearance to trigger coils, plus if it wobbles it's not likely to affect timing much.

[Linked Image]


These carbon/titanium mufflers came today, 51mm straight through, nice deep tone, at idle frown

[Linked Image]




mark
Re: Cam timing question with A65 750 [Re: Mark Parker] #749970 09/21/18 10:32 am
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What does it rev to Mark?


beerchug
Re: Cam timing question with A65 750 [Re: Mark Parker] #749983 09/21/18 11:29 am
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Did the mufflers help with any reversion ?


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Re: Cam timing question with A65 750 [Re: Mark Parker] #749994 09/21/18 12:11 pm
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It revs to about 3000 at the moment, that's the problem. When it actually runs I expect it should have power up to 9000 or so. Mufflers made no difference. It was fine as a 650 with these pipes and similar size ports. Ign timing is not over advanced.


mark
Re: Cam timing question with A65 750 [Re: Mark Parker] #749996 09/21/18 12:24 pm
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Nothing to do with there being no long "Stack" or airbox attached to those carbs is it Mark ?

That said if the setup is the same as when it was a 650, the bigger cc shouldn't be hindering it, it could make it a little richer though as the pulse on the jet will be stronger, when i first started doing my small port mods, I had to reduce jetting size as the mixture became too rich.... all because the signal on the jet was stronger.


beerchug
Re: Cam timing question with A65 750 [Re: Mark Parker] #749998 09/21/18 12:38 pm
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I might have missed something here but are you getting that condition when opening the throttle statically or actually riding? Sounds like static and just opening throttle. I would be tempted to go to smaller needle jets. At light throttle I would not suspect 'reversion' just too much gas. Assuming your ignition is sorted. Have you had a chance to 'whip it open' on the road yet? Cheers, PRT

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