...hello, an hour ago I took the road to perform that dangerous main jet tuning (also attacked by killer bees! now I have the head inflamed in some spots)
-Spark plugs are Champion L87 due to work better in my bike than the L82. -Magneto is a Hunt. -Carburetor is a 626 with the thicker O ring that I brought a year ago with homemade filter due to no space in the 48s models for a proper filter; however, never had a problem with this filter and I am using it since 20 years ago. -no goo on the jets.
-In this country there are only 2 types of fuel. One is "premium" for the new cars then the other is "super" for the rest. About 10 years ago was "normal" that was what I used for the bike. I think super is 95 and the other is 97 (or 92 and 95). Discontinued one was 85 or 87 I think.
---spark plugs were right and also look good in the other stages BUT I still have that kind of banging in low gear. I could tight more the carburetor nuts due to possible air leak...but I do not know.
I did not had this banging with the previous 626 but did not idle; this one does not idle too. The bike only idles when hot; so if I touch a bit the screw after a short ride the idle is too fast.
Loud banging in the exhaust while slowing down with the throttle shut is usually caused by an air leak where the exhaust pipe fits onto the cylinder head.
Weak mixture at small throttle openings can cause a less-loud banging or popping in the exhaust. The cause can be an air leak into the inlet manifold. Test by spraying WD40 or similar on the joints while the engine is idling. Other causes can be maladjusted idle mixture screw or a too-big throttle slide cutaway.
...do not see how I can go in top gear with the throttle almost open or the other way around...hence that I mentioned that the fact occurs in low rpms so low gears...anyway; I do not have tachometer but around 1500-1800 rpm s. Around 1/8 open throttle, so like Mr Healy is saying could be a weak mixture, but the screw is in the 1 1/2 turn. -I do not have an erratic slow running.
I think what is being pointed out is that if your slide is too rich, you compensate by making the idle air too lean. It seems your problem is occurring on the overlap of these "two carbs" within a carb. Also your cutaway may be stamped with a number, but that number is just an estimate of the actual cutaway. You could try the 3.5, and you can emory paper your 3's to 3.25. IOW's you can fine tune the thing.
Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
Exactly 1 1/2 turns is a starting point. It may be correct or it may be 1/8 or so of a turn one way or the other to be correct. That's where the fun comes in on these old turds. No two carbs are the same. No two cylinders run the same. Experiment, experiment, experiment.
1968 T120R 1972 T120RV Any advice given is without a warranty expressed or implied.
No sarcasm there at all Reverb. You've said you are confident that the mixture/plugs are good at all ranges The weird thing is that this "banging" only occurs in gears 1 & 2. In general, carb tuning faults are not gear or rev specific, they are throttle position specific. That is why I questioned the gearbox, as someone else already has.
So my difficulty is to come to a theory as to why this happens only in gears 1 & 2. Could it be that only in those gears are you able to trickle along on the tiniest whisker of throttle at very low revs?
If so, then it points to something that causes an occasional misfire at those conditions. This could be:-
- weak or mistimed spark at very low revs. I know nothing about Hunt magnetos, so I won't even explore this.
- wrong (probably weak) mixture at very low throttle openings. I think this the most likely. It would be at such slight throttle that the mixture is strongly governed by the pilot jet circuit. A weak mixture here could be a partly blocked jet (or the tiny pilot holes) or too much wear clearance between slide/body. Alternatively, an over-rich mixture here could be caused by an enlarged pilot jet, due to clumsy unblocking.
There seems to be some doubt about the bike's ability to idle. I think this is the best area to investigate.
With the engine fully warmed up, can you obtain a steady, relaible, fairly low idle? Take special care that the setting of the air screw gives the highest speed, ie that turning it half a turn in or out will lower the speed. If you can get a nice idle, how many turns out from fully in is the air screw? By turn, I mean 360 degrees. If you get this nice idle, go for a test ride.
If you can't get a low, steady idle, or if the air screw is far away from 1.5 turns out (anywhere roughly between 1 & 2 turns would be ok), then the pilot mixture needs further investigation.
As mentioned, The pilot could be, and most likely is partly clogged, unless you have cleaned it lately with #78 drill bit. Get one, try it. It is a treat what a difference it can make.
I was afraid at first of breaking it off inside and not being able to retrieve it. But even though my hands are not the steadiest, it is easily done if you take your time and be patient. I practiced first on an old swap meet carb. That gave me confidence. You can do it with the carb still mounted on the bike. Just get a seat to sit on. Offer the bit to the jet (firstly set the bit in a tube of some sort) sort of poke around until you have found the entry orifice. This can be the hardest part. Once I find it, I very gently push it in until I feel it bottom, and gently twirl it bit as you withdraw. Check the bit. You will most likely see some black, or maybe grey/white if you use leaded, (which came out of the old carb), that provides more resistance than the black, which you can hardly feel.
At the end celebrate, you have just resized the carb, and done the fastest Big tune up possible, with a tiny tool.
Last edited by btour; 12/22/176:54 pm.
Bob, Lifetime bike: '71 T120R, bought in '71 at Ken Heanes, England.
...hi Koan58, see with other point of view, without thinking in the gears but think in going slow (hence low gears, hod do you go slow in top gear?) I did not said that the mixture is good in all the phases, only said that the plugs look right in all the phases (hence...)
Looks more carb problem than a misfire.
Regarding idle, yes, only idle when hot but if I put lot of kms at top speed, the idle is too high (more than 1000 rpms) If I move the throttle screw, normalize but then will be too low when engine be less hot or cold shutting off always.
Hello Btour, I bought a non premier carb but somehow this body is a premier carb (may be all the new 626 are like these?) so have the other screw in the opposite side and the conduct is clean; also I passed a guitar string.
Hello Mr Healy, as mentioned is an intermittent popping at around 1500 rpms or so with a "fixed" throttle near 1/8 open. No popping sound on deceleration at any stage. All carb parts in good conditions; open exhaust pipes and air filter. in 20 years may be this is the first time doing this but I am not totally sure but always with the same set up except for this new carb bought a year ago or like that.
Try screwing each pilot air screw in one 1/4 turn then report back on symptoms.
1.5 turns out is a general starting point, in your case this seems like it might be too weak.
The fully warmed up setting is the correct one, ( max idle speed from pilot air screw settings , erring a little to the rich end of the screw setting) if the bike struggles when less than fully warmed consider fitting choke slides.
71 Devimead A65 750 56 Norbsa 68 Longstroke A65 Cagiva Raptor 650 MZ TS 250 The poster formerly known as Pod
Hi Reverb, I speculated that the problem only showed in gears 1 & 2, because only in those gears could you trickle along on the tiniest throttle. I find on my 650 I can do this on a level road, in 4th gear, at ~25 mph (which is ~1500 rpm), with just the slightest opening of throttle when the speed drops to ~20 mph. (dugga dugga dugga) This most certainly isn't using anything like 1/8 throttle, more like 1/64. 1/8 throttle would be an easy 40+ mph on a level road. So why can't you do 1/8 throttle in gears 3 & 4 on a level road, and get the banging then? When the banging happens in 1st gear, at low revs and small throttle, can you feel the jerkiness of the bike? In which case it is a misfire. Or is it still smooth while making the banging? In which case I can only imagine unburnt fuel is getting into the hot exhaust? Is the banging worse when the engine is cold or hot?
Exactly what engine are we dealing with? A 5T or T100, what year? I guess an iron head with the "L" plugs.
Reverb, where is south cone? I'm guessing it's a warm part of the world? If so, why do you need hotter plugs? It is possible that you are hiding over-rich mixture fouling?
On the carb - wouldn't the original be 15/16" bore (~24mm)? Having replaced it with 626 (26mm bore), it will tend to run slightly leaner (lower venturi speed). I wouldn't expect this to be critical, but others may know better? Certainly make sure the gasket carb/flange doesn't intrude.