Pinto Kit car with bike carbs setup

[pringle1988]

Hello all im looking for some advice here.

I bought a kit car a couple of years ago now (pretty sure its a locost). It came with a 2L pinto lump in it running motorbike carbs which are mikuni and have 4 venturi? Basically 1 for each cylinder.
I have never been able to make this run correctly and seems to now be running on 3 cylinders. I have done some research and have been advised it is best to get these setup on a rolling road. Is this correct?
It also backfires now and again sometimes out the exhaust (with flames) and sometimes out of 1 of the trumpets of the carb.
Could the needles be the incorrect size? Sorry for the questions im pretty new to the world of carbs haha.

I have checked for a spark and made sure the timing is correct and these seem well. I can also see the fuel being drawn into the cylinders when I look into the carb.

Any help would be appreciated thanks

[TheRealFingers99]

pringle1988 said:

I bought a kit car a couple of years ago now (pretty sure its a locost). It came with a 2L pinto lump in it running motorbike carbs which are mikuni and have 4 venturi? Basically 1 for each cylinder.
I have never been able to make this run correctly and seems to now be running on 3 cylinders. I have done some research and have been advised it is best to get these setup on a rolling road. Is this correct?
It also backfires now and again sometimes out the exhaust (with flames) and sometimes out of 1 of the trumpets of the carb.
Could the needles be the incorrect size? Sorry for the questions im pretty new to the world of carbs haha.

I have checked for a spark and made sure the timing is correct and these seem well. I can also see the fuel being drawn into the cylinders when I look into the carb.

You might start by investigating the cylinder that's misfiring.

First, find out which cylinder that is. Often the easiest way is to pull off one plug lead at a time (and only one): the misfiring cylinder will be the one which has the least effect on engine speed.

Remember that the cause of the misfire can be mechanical (a sticking or damaged valve, poor compression), electrical (plugs, leads, coil(s) or dizzy), or fuelling. But as it's actually backfiring through the carb, I'd guess it's mechanical, a valve issue. Compression test, dry and wet.

It's unlikely, I'd think, that you've 4 carbs, 3 of which are set up well, one of which is miles out. You might compare the jets, needles, etc. between the "bad" cylinder and one of the good ones.

If, at the end of all this, you want to set the fuelling up properly, by all means take it to a rolling road, but make sure that the RR has experience with carbs and access to Mikuni parts.

[PaulKemp]

First point of call would be to balance the carbs
Use a an air flow meter the ones that are cylindrical with a tapered rubber bung, stuff it down each trumpet in turn and note numbers for each
The centre right carb is usually the master connected to the throttle cable the adjust others to match.
Most of the running is done on the jets with the main jet and needle position for wide open throttle.
A rolling road tune ( one than understands bike carbs) is the only true way to set them properly.
You could attempt it at home but you would need a gas analyser or a lambda probe with gauge
I had the same issues as you and cured most with balancing and needle clip position, still needs a proper rolling road session though

Edited by PaulKemp on Tuesday 12th August 06:39

[vx220]

Company called Bogg Bros is widely regarded as a good place for bike carbs

Anywhere near you?

[Mr2Mike]

Does it run on three only at idle, or doe the fourth cylinder never fire? If it's the first, then it's very likely a blocked pilot jet. They are so tiny that it only takes a minute amount of crud.

[Steve_D]

vx220 said:

Company called Bogg Bros is widely regarded as a good place for bike carbs

Anywhere near you?

And/or Chester Sports Cars.

Steve

[TheRealFingers99]

But why would it be "firing back"?

I still suspect a mechanical issue.

[Mr2Mike]

TheRealFingers99 said:

But why would it be "firing back"?

Absolutely classic symptom of running weak, which would be caused by a blocked jet.

[pringle1988]

Thanks for all the replies everybody. I forgot to say I have done a compression test in the past and all cylinders came back pretty equal and within tolernaces for the engine.
Im going to do another compression test tonight and see what the results are along with rechecking for spark etc.

With regard to running on 3 at idle, this could be right, as soon as I press the accelerator even the slightest bit the engine goes from running like its trying to escape the car to pretty smooth although it sounds a little off beat still. I just put it down to faster engine speed smoothing out the bumpyness. The manifold for the exhaust has been completly custom made so not sure if this can cause it to sound a bit off beat?

Chester sports cars is a little far to go but Bogg Bros although quite far is a lot closer. Im based in Durham if that helps and worst comes to worse and have to get it looked at.

Thanks again.

[LLantrisant]

did the car every run roperly with those carbs?

what info did you got from the former owner?

valve clearances (sports camshaft fitted) and ignition timing are correct?

[pringle1988]

LLantrisant said:

did the car every run roperly with those carbs?

what info did you got from the former owner?

valve clearances (sports camshaft fitted) and ignition timing are correct?

The car in my ownership has never run well, to be honest ive hardly used it though as I kind of got annoyed with it not running well. The former owner didnt give me any info on it. When I went to buy the car I was more into owning a kit car really and basically had rose tinted glasses on. Ive checked the timing and it is spot on but I havent looked at the valve clearances.

I dont know if this is a stupid idea or not as im a bit of a novice with carbs but if it was something to do with the idle jet as said above could I spray petrol into the carb to see if it the runs on all 4 at idle?

[TheRealFingers99]

If it's firing back, I'd think it ill advised.

[PaulKemp]

Get it set up properly
If you have little knowledge about carbs your just working in the dark

If you do decide to try something yourself start with the basics
Spray WD40 round manifold face and carb connection rubbers while engine running, if revs rise you have an air leak
Check throttle action, are all throttle plates evenly closed and opening together,
Balance carbs
Some bike carbs have idle air screws exposed some are sealed

There is no point in looking for other reasons until you know the basic set up is functioning correctly

[Mr2Mike]

A quick check is to gradually "choke" the carb air inlet on the misfiring cylinder using your hand, or e.g. a piece of wood. This will richen the mixture, so if that cylinder starts running you know what the problem is.

Sometimes it's also possible to clear a blocked jet by completely choking the carb briefly with the throttles opened (don't use you hand for this though, the vacuum will be very high). I've used this method very successfully on a number of old bikes and lawn mowers etc.

[pringle1988]

Thanks again everybody, Ive been back working on it again tonight.

I took each lead off individually and noticed it was actually on 2 cylinders only, untill I pulled the choke slightly. I now have adjusted the screw and have it running on 3. (The 3rd would always come on with throttle but not at idle). Its cylinder 4 thats my issue. Ive rechecked for a spark and thats good, but it does not fire at idle or with throttle.
I have noticed that the the black bit of plastic connected to the needle and vacuum vibrates and sort of chatters up and down. So I checked the vacuum seals at the top and all is good.
I have re done a compression test and im getting results between 130 and 110 PSI. To me this seems low but im used to more modern engines. Is this ok?
Does anybody have any access to a Mikuni carb diagram as I can see 3 screws but there are 4 carbs. To be honest its about time I seek professional help haha.

[one eyed mick]

Getthe car to Bogg brothers and have a set up done they are the best in the north east ,mony people in Rhocar swear by them [not at them ]

[PaulKemp]

Compression test should be done with throttles open or your trying to drag air through closed throttle plates

[LLantrisant]

another idea:

if you have space or already a bonnet cut-out, go back to a downdraught carb...well jetted its not far away from your carbs and much, much easier in terms of maintenance, reliability etc...

Edited by LLantrisant on Wednesday 20th August 21:44