BIG carburation problems after installing 'Hot' cam

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Hi Guys,

After what seems forever trying to resolve a carburation problem on my custom built big bore street scrambler I've decided to see if anyone on here can offer help.

So, I have a 270 rephased motor with 750 big bore kit, running a pair of VM34's, with an after market 2-1 exhaust and a UK made electronic ignition. After experiencing very poor performance I checked the profile of the cam, only to discover it had 93 degrees of overlap, so most of the fresh fuel was disappearing out of the exhaust. To correct this, Smedspeed in the UK made me a new cam (their 3.5 'hot' street cam). After installing the cam and carefully static timing the new electronic ignition to the recommended 38 degrees BTDC fully advanced (Smedspeed and ignition makers recommended setting- subsequently strobe timed to check advance operation - appears to be working as it should) the bike now suffers from MASSIVE misfiring from half throttle upwards. I'm running B7ES plugs and they look extremely lean, being very white at the electrode.

I have tried changes to the main jets (150 through to 190s), pilots (20 through to 25s) and gone from the fitted 159P6 needle jet to the next richer P6. I've tried ever combination of needle position with the various jets too, but no matter what I change there is virtually no difference in the degree of misfiring or where its ocurring, which is half throttle upwards, right through to full throttle. I'm running 2.5 slides, which are quite rich, but you'd never guess looking at the plugs.

A friend of mine who is somewhat of a Triumph guru and has been in the business building and tuning customer Triumph T120s, T140s and every other form of Triumph motor, has been working with me on this for a long while now and it is perplexing him. He suggested trying a pair of MK1 Amal concentrics, Amals being his specialist knowledge carb-wise. So we swapped out the Mikunis and went for more and more richer jet set ups with the Amals, right up to seriously rich 250 mains, but the misfiring was no different and the plugs always look extremely lean.

I am now having serious doubts about whether the problem is the carbs or the aftermarket 2-1 exhaust. I understand these exhausts can cause significant problems for performance cammed motors. So I am hoping that after reading this, someone out there with a wealth of experience in tuning these rephased motors with VM carbs can give me their thoughts on what is going wrong here. With the new cam, new rockers, new electronic ignition I've invested heavily in this motor but getting no benefits, so I'm hoping someone can help before I throw in the towel and sell the bike, something I'd rather not do.
 
Have you tried going the other way with the jetting, despite the plugs looking lean, if there is a large overlap it normally spits it out of the carb not the exhaust in effect making it have more fuel as it is the pulling this fuel in, I have a massive overlap on mine and have had similar issues, one being fuel supply as on opening the throttle it drained the bowls faster than the fuel was getting there.
 
Have you tried going the other way with the jetting, despite the plugs looking lean, if there is a large overlap it normally spits it out of the carb not the exhaust in effect making it have more fuel as it is the pulling this fuel in, I have a massive overlap on mine and have had similar issues, one being fuel supply as on opening the throttle it drained the bowls faster than the fuel was getting there.
I no longer have a large overlap issue. I had a new cam cut to Smedspeed's suggested profile. I think, if memory serves me correctly, the overlap is now back where it should be at around 43 degrees.
 
Just as a point of reference, Heiden used to recommend 148 mains in Dellortos I run mine on 132s there was very little change in colour on the plug but sorted the misfire,
 
I can't claim a wealth of experience specifically with re-phased engines but... massively lean and misfiring over half throttle with little to no reaction to richer jetting suggests a possible vacuum leak??? I have been tripped up by this before, by carb vibration isolators that developed leaks with revs. You mention throttle position but not rpm, at what revs does the misfire start to occur? Does it pull cleanly up to that point?
 
I believe I ended up about 195 mains on my big bore with vm34s.

Stock cam and open 2 into 2 pipes.

- you have gas, it’s getting gas…

Anyway to hook up your ignition backwards??

If there is a battery, it’s good and charged and ignition isn’t super finicky with voltage?

All your grounds are solid?

It breaks down all the same if you barely roll the throttle?

If you pull the choke it does nothing??


just ideas to think about.

*90% of carburetor problems are ignition haha.
 
Unfortunately they have offered little real help. Just a quick suggestion as to a possible needle jet and needle choice to try.
Oh dear. That isn't great. Glad I have never spent any money with them then. Good luck sorting it. Kind of odd that two different types of carburettor give similar results.
 
The bike has an Acewell
I can't claim a wealth of experience specifically with re-phased engines but... massively lean and misfiring over half throttle with little to no reaction to richer jetting suggests a possible vacuum leak??? I have been tripped up by this before, by carb vibration isolators that developed leaks with revs. You mention throttle position but not rpm, at what revs does the misfire start to occur? Does it pull cleanly up to that point?
The bike has an Acewell digital rev counter which by all accounts isn't a very smooth output, which is the case with mine, erratic, so cannot be relied on. This isn't an airleak issue, as it has new gaskets at the head/manifold and it uses very thick flanged aluminium intakes which have been checked for flatness.
 
I believe I ended up about 195 mains on my big bore with vm34s.

Stock cam and open 2 into 2 pipes.

- you have gas, it’s getting gas…

Anyway to hook up your ignition backwards??

If there is a battery, it’s good and charged and ignition isn’t super finicky with voltage?

All your grounds are solid?

It breaks down all the same if you barely roll the throttle?

If you pull the choke it does nothing??


just ideas to think about.

*90% of carburetor problems are ignition haha.
Lots of valid comments. Thanks. The ignition is brand new, built especially for this bike by Pipes N Stuff. Timed up exactly to the degree. No, can't get timing reversed, and it has been checked many times. Battery is a new lipo, good pma charging system. Grounds are all solid. And doing a flow test on the 'hi-flow' fuel tap shows its delivering 0.4L per minute (after the in-line fuel filter) which is plenty.
 
Oh dear. That isn't great. Glad I have never spent any money with them then. Good luck sorting it. Kind of odd that two different types of carburettor give similar results.
Yes, that's what is leading me to think this may be non-carb related, but it isn't cam or ignition timing because these have been checked to death.
 
Lots of valid comments. Thanks. The ignition is brand new, built especially for this bike by Pipes N Stuff. Timed up exactly to the degree. No, can't get timing reversed, and it has been checked many times. Battery is a new lipo, good pma charging system. Grounds are all solid. And doing a flow test on the 'hi-flow' fuel tap shows its delivering 0.4L per minute (after the in-line fuel filter) which is plenty.

Do you have a way to confirm the ignition is functioning properly??

I’ve seen my fair share of ignitions (of various makes) be duds out of the box.
 
Do you have a way to confirm the ignition is functioning properly??

I’ve seen my fair share of ignitions (of various makes) be duds out of the box.
I'm not sure how I can check that. I've set it up as instructed statically, on left cylinder, and then put the strobe on it and revved it to around 4000rpm and it appears to be advancing as it should. Haven't checked right cylinder as I assumed, rightly or wrongly, that the other sensor is set correctly at 270 degrees.
 
I'm not sure how I can check that. I've set it up as instructed statically, on left cylinder, and then put the strobe on it and revved it to around 4000rpm and it appears to be advancing as it should. Haven't checked right cylinder as I assumed, rightly or wrongly, that the other sensor is set correctly at 270 degrees.

It would be wise to check the other sensor.

Also as stated by another poster what’s the voltage when running, and being revved?

Does voltage drop? If it does that could be an issue for your ignition. Some are much more prone to voltage breakdown than others.

They should also have specs to measure the actual ignition by. Maybe contact the manufacturer.
 
*90% of carburetor problems are ignition haha.

Do you have a way to confirm the ignition is functioning properly??

I’ve seen my fair share of ignitions (of various makes) be duds out of the box.

I’ve had more bad running issues that were ignition related than anything else. I even had a new generation Triumph Bonneville that had factory electronic ignition problems that I never could fully resolve. And the last generation Pamco ignition on an XS650 that had a bad electronic advance that would break down as soon as the motor got hot.
They can make you chase your tail because often the problems are intermittent.
 
I’ve had more bad running issues that were ignition related than anything else. I even had a new generation Triumph Bonneville that had factory electronic ignition problems that I never could fully resolve. And the last generation Pamco ignition on an XS650 that had a bad electronic advance that would break down as soon as the motor got hot.
They can make you chase your tail because often the problems are intermittent.
I can vouch for that; I'm still chasing an intermittent mis-fire on Taffy after two years and three different TCI / Gonzo set ups.
At my witts end with the bike I love!
 
I can vouch for that; I'm still chasing an intermittent mis-fire on Taffy after two years and three different TCI / Gonzo set ups.
At my witts end with the bike I love!
Is the trigger and magnet(s) working properly? It would explain why two or three different igniters aren't working properly.

(I previously owned a Hinckley Bonneville. Faulty ignition triggers were extremely common. Often only giving a fault after serveral miles and then working normally again for several more miles before going faulty again).
 
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