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Big Al in York, gavin eisler, JER.Hill, Mark Parker, MarksterTT, Nick H, NickL, PEH, pushrod tom, Stuart Kirk, Triless
Total Likes: 25
Original Post (Thread Starter)
#841500 03/01/2021 6:51 AM
by NickL
Running up after fitting new cam etc, old moron tweaking the carb........
Liked Replies
#847863 May 3rd a 03:18 AM
by Mark Parker
Mark Parker
Plenum effect is back, if I move the box around over the thing running the vacuum moves up and down, so the effect helps when it crowds the intake a little.

So through the std 40.5mm 152cfm ,about, and 157.3 through the seat opened to the 42mm valve, both with the 34mm carb. If my base calculation is correct. So a stock valve with 34mm carb should perform really well with this port and I will do a head.

I cannot do the 42mm seat properly without the proper guide fitted and K-lined which is annoying. I cut it so I could mark it with blue, but hopefully will be close. The worst thing on these is an automotive cutter to cut 3 angles at once, as the hemi insert then gets cut in with a step to get a top cut, the deeper the worse.

The plenum effect gives 171 with no carb and radiused entry. I guess the more it reads whatever it actually is the better. And it reads consistently and the comparison is good. But now is pulsing a bit. And higher lift is worse than .350" it makes more noise at .388" but doesn't flow more. so maybe the air is leaving an edge and breaking up at higher speed, and its faster but blocked by some turbulence?

Through the carbs it's at least + 45% on stock. I'll take some photos when I change it over for the other side.

These little welded manifolds are round 34mm tube squashed at the head end to be 30mm high and whatever wide the threads are helicoiled to smaller dia for these screws to fit.
[Linked Image from]

They are within 2cfm and one is better further open for some reason. One side broke into a valve spring recess frown so a patch. The SRM race springs fit shorter so a washer can protect the little patch.
3 members like this
#844911 Apr 2nd a 07:16 AM
by Twin Pot Phil
Twin Pot Phil
Nick, I have never quite understood the CRMC, 'they' want to reproduce 'of the period' racing, but allow not 'of the period' engines and modifications - like the oversize triples you detail.
I remember in the 80s having a conversation with 'an official' and when asked about thoughts I said overwhelmingly officious and overbearing, he was surprised at my statements! - only changed later when Ian Johnson (spelling!) became involved, more relaxed and friendly with common sense, better grids. Apparently has now gone back to the way it was - pot hunters throwing money at machinery.

PEH, Cadwell is fun because it is so narrow - it demands 'focus'. I remember Dave Hallam on his TZ750 outfit failing to turn right at park corner, the front hub ended up buried in the gearbox.

Oulton Park events are now limited due to newcomers and noise limits. The government refuses to grant 'grandfather rights' to circuits every time they are asked.

Chris Vincent's TT outfit was initially an A7 based engine, he later moved on to BMW engines to go faster. For mainland Open class events he used an A65 engine.
2 members like this
#848658 May 11th a 12:14 PM
by Blown Income
Blown Income
Originally Posted by pushrod tom
We are hoping to see Chris this year. I did the dyno work with him and was really amazed at the numbers we saw at only 6k. With your head it should make some serious record breaking horsepower.. PRT
PS, His bike is beautiful!
Hoping to see Tom and Mike this summer for some well over due tuning, the A65 LSR Lightning is sitting right up front starring me in the face when I enter my shop. I wouldnt be surprised if the 34mm carbs will need to be replaced with larger ones. As soon as I finish up a Victor I'm working on the Race bike is on the lift to check compression after sitting for a few yrs, change the oils and fingers crossed, fire her up.
2 members like this
#848727 May 12th a 06:55 AM
by Mark Parker
Mark Parker
Firebird's original head, stock valves and ported to 34mm PWKs.

This is interesting because I tested side by side with a stock head using the same valve, 40.5mm. I assume the stock head is 109cfm minus carb. With carb then 106.5cfm. The ported head is 155cfm through the 34mm carb and same valve. Or some where near it. 45.5% approximately more air. And importantly much faster. And in the Wallace hp calculator for hp based on flow about 45.5% more powerful. Std 106.5 works out at around 50hp. On their mildest spec Street/strip. High comp, 11 or 12-1 should put it in race motor mode and hp estimation. It would be interesting to see. 750cc brings rpm down.

The 42mm valves are about plus 50% through the 34mm carb.

This shows ports at stock valve size, stock 40.5mm valve, 42mm and 44.5mm.

[Linked Image from]

[Linked Image from]
2 members like this
#853473 Jul 11th a 08:01 AM
by NickL
Well, just got back from a chilly meeting. (for us soft Queenslanders anyway!)
3 races Saturday, 2 wins and a second.
3 races Sunday 1 win 1 3rd and one did not finish, he broke his lanyard and
the motor obviously died.
He was struggling as the motor was only the 650 and he was up against the big norton and the triumph again.
Also found the battery was flat after his 3rd finish.
Had fitted a new rear tyre which was way to big in overall diameter and it upset the bikes geometry by a mile.
Still, he is happy with the baby engine as his lap times were only 0.6sec slower than the 740.
It will make a very usable spare engine. Gearbox was spot-on too.
Would have been nice to have time to have set the carb up properly as it was fluffing at lower rpm, i think
the acc pump jets are a bit small. Seems a bit more difficult to get it right with this engine than the larger one.
State titles at Morgan Park in early September next up.
Found a dyno not too far away and are booking some time, hoo-bloody-rayyyyy finally.
2 members like this
#841701 Mar 2nd a 10:48 PM
by NickL
No worries, i'll get Matt's wife to video it. I may even take it 'round for a few laps myself
if my wife lets me.... It's only a club weekend meet so pretty layed back. It's not as if i'm
going to try and win is it????
1 member likes this
#841586 Mar 2nd a 12:16 AM
by NickL
Sorry about the feet....................
1 member likes this
#842768 Mar 14th a 01:01 AM
by NickL
1 member likes this
#843092 Mar 16th a 08:49 AM
by NickL
The primary is used as a plenum the timed breather is blocked off.
That 3/8 pipe is the only breather, it seems to work fine. Never used
a reed on any of my race engines but all breath into the primary so
much larger expansion area.

Mallory was a great circuit, better before the silly 'bus-stop'. Had heaps of
fun there.
1 member likes this
#843105 Mar 16th a 12:40 PM
by Tridentman
Mallory was my local track-- I lived about 15 miles away in Rugby.
In my early days used to ride there with friends--the ride there was quite fast but the return ride after being inspired by the racing was pretty hairy.
My favorite spot to spectate was on the exit from the hairpin--you could see the bikes coming up to the hairpin and the outfits drifting around the hairpin before drifting into the Elbow.
Worst sight was when I was spectating from the entrance to the hairpin--Fritz Scheidegger had brake problems and didn't slow down or take the hairpin--went straight on, hit the top wall of the hairpin and was thrown high into the air like a rag doll. Terrible loss as he was a really good rider.
Ah well--life goes on!
1 member likes this
#847258 Apr 27th a 06:04 AM
by JER.Hill

If you can machine the crankcase to centralise the alternator/out rigger carrier, then you can also increase the OD to allow you to carry larger engine sprockets. Three counter sunk hole to fit the mounting to the crankcase, then another three holes to lock your out rigger into, this way you can give it clearance for the larger sprocket! To make it one piece there is going to be a lot of swarf, so might be a case of make the crankcase mounting and weld the three leg to carry the out rigger on.

1 member likes this
#848299 May 7th a 03:19 AM
by Mark Parker
Mark Parker
More lift on my 883 made it better, moved the power up. As an 810 it would pull well to 7500 on the data logger about 55hp as 883 those 38mm ports and 44.5mm valves ran out around 6000-6500, and it died, though it had lots better power while it worked 65hp. Offset rocker buttons added about .020 lift. And it was better. And I was able to get 74hp. It pulled to 7,000. But changing the head made 40cfm difference with the port the same size and it's still pulling at near 8,000 and the guy wanted to give it 1000rpm more to see where the graph went over. I wasn't that curious.

It would be interesting to run it again and see what richening it, and better mufflers did, but I don't want to blow it up doing rpm it will never see. A better cam may make it better, if it was easy to swap and cheap.

It's just I measured the std head and extra lift doesn't do much, but different heads can act differently how they respond. And duration makes a difference.

From what I've seen the A65 stock motor isn't restricted by the cam so much as the head. It's not a direct relation ship between cam spec and rpm levels when cylinder head flow is changed appreciably. More air allows higher rpm. If the head is at a good place, then testing cams can show what spec works best.

Why I'm interested in the difference between how the current rig goes with the two different cams. Is one noticeably better? And it's not particularly peak hp but drive.

I'm thinking I should give the Firebird a short dyno and hope the mixture is close and it stays together. It's not like I don't test it on the road but it makes me nervous. Like going home, Deb where's the bike? I need the trailer and degreaser. Should get Ben to take it up.

The head on the 883 didn't make much difference in the mid range, the Firebird feels different from when you take off. And seems to totally transform it in a 650cc kind of way. And it's hard to quantify without a stock one to compare, so I need some sort of test. The SV is out of rego so I cannot do rollons with that which would be easy.
1 member likes this
#848373 May 8th a 01:12 AM
by Mark Parker
Mark Parker
Place in Melbourne done once and no problem. Place in Qld is much cheaper, however it came off twice and the second time destroyed the piston, so cheaper to pay more the first time in Melbourne than twice plus piston.

John Hill in the UK does alloy cyls and Nicasil bores to your piston size. It's more expensive to machine and have bores done here. A JE forged piston can run .002" mine are .0025". I've not tried the 76mm A65-70 pistons EdV does from JE, Just 79.5 and 80mm B44. They work great with the long stroke because the skirts are short. They really need longer rods. I had a taller alloy block cast but never used it.

[Linked Image from]

This is a sketch of the XR750 port. So I'm trying to improve the bottom turn and make everything smooth. And stop break aways and turbulence, the turbulence I think it needs is entering the cylinder and its what it is.

[Linked Image from]
1 member likes this
#850626 Jun 4th a 01:48 AM
by Mark Parker
Mark Parker
So with the 34mm manifold and pwk carb on a big 44.5mm valve head, flow maxes out at 177cfm. This manifold doesn't fit right as the top of it is blocked. It's just a matter of doing a head where it lines up properly and making the port all the correct size and shape. At the moment through that 34mm manifold with radiused entry it's 191.3cfm, a bigger carb will not loose so much but a bigger port loses response.

It looks like making a head with the 44.5mm inlet and 34mm port could be lots of fun, if it can retain the midrange response, and I do not see why not. Low lift flow isn't quite there yet but the seat and port can be better. I need 4 vacs going to test this and higher test pressure. It's just flow calculated from testing a std head. The smaller valves are perfect for stock sized or long stroke motors, but this may be the thing for a big bore 750 race motor when you want strong midrange drive and don't want to exceed 9,000 particularly. It might actually be that carb and port size do not give much more flow than smaller which increases speed and power. I've run my 883 on the dyno once and had a best of 85hp at the wheel and lots of pulls around 83hp but adjusted nothing because the main jets were so lean. So I don't really know what is possible for that big motor, in Vizard's book he talks about getting a motor to finally give 145hp on a dyno and then spending 3hrs fiddling with it to get 170hp. So maybe 95 at the rear wheel is possible with the 883 set up right, if it stayed together and if there was any point.

Problem is I only have a couple of good valves. Which are cut down nitrided Jaguar that I don't seem to be able to get any more. Valve seats need to be central to get these to fit as well, but I think it's worth doing on a 750big bore with lots of compression and a cam. The idea is to make good hp to 7500 and be able to use more at higher rpm if necessary.

Just to add later T/bolts with 40.5mm valves have larger inlet seats so don't need the expense of fitting new seats.

Edit; looking at the seat it's very ordinary but making a better fitting manifold gives a better idea of what is possible. Through the carb at .428" 180.5 .385 178 .350 174 .300 162 if that stays close and is anywhere near accurate, on a finished 34mm port it would be devastating. A 38mm carb and port is only 184 so unless that was increased it's no advantage probably a loss.

[Linked Image from]

The valve is holding the 34mm tube that is squashed oval at the head in place jammed against the cap screws. This head on an RGV needs curved manifolds so Ben is looking for a good Thunderbolt head.

From Vizard's book a std 35.8mm exhaust valve is a close match to a 44.5mm inlet in a 9.5 to 12-1 engine for best power. MAP have 2X B50 44.45mm black diamond swirl valves $40us each.

Do you know what lift @ the valve the SRM cam has Nick?
1 member likes this
#852278 Jun 24th a 01:15 AM
by NickL
Well, ran the old thing up on Sunday with the 650 motor fitted and the head i modified
fitted with the weber. Seems good and sounds lovely, all ready for the 'Big-Chill' meeting
in July.

I've told him to take care of my
1 member likes this
#854053 Jul 18th a 10:48 PM
by NickL
One of the races up at the goat track circuit last week.
1 member likes this
#866836 Dec 20th a 08:35 AM
by NickL
Just a photo showing the simple end feed arrangement i use on a65's.
It maybe helpful if someone is contemplating doing the job.
1 member likes this
#867179 Dec 24th a 11:29 AM
by Twin Pot Phil
Twin Pot Phil
Originally Posted by NickL
Was -1 deg on Sunday morning until about 8am.
After living here for over 20 years i have lost my pommie cold weather hardness.................Nick
Harumph, only 20 years, standards old chap, standards. Don't let the natives drag you down, stiff upper lip now.
(runs and hides in a very dark corner!!!)
1 member likes this
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