DRLA 36s and how to stop my bus backfiring?

Pretty sure pump jets are on the side of the carb and sort of enter in through the Venturi. edit you can see them in your photos above. Brass bit sticking out through the Venturi into the big hole above the butterfly 'flap'

 

Sometimes what is stamped on the idle and main jets may not be correct.

People drill them out to make them bigger, the convention is to scratch through the numbers to show they have been altered.

So yours are probably what they say they are.

 

I'd find a cheaper rolling Rd as I've never paid even half of that.

They will be able to measure air fuel ratio and 02 (?) throughout the rev range, not just at wide open throttle.

They will be able to change jets to the optimum for your engine and adjust the ignition timing etc.

Make sure your valve clearances are spot on before you go.

 

I've got a diagram somewhere I can post later

 

Your Venturi are too big and your mains are too small for starters ..A rolling road will most likely not get you the results you want as they most probably won’t have a ton of different size jets . My 36 run so sweet on my 1700 type 4 but I used a lamda sensor to get them right

 

One thing you can check easily enough is the idle jet function on each cylinder.

I'd suggest having a friend there and some rubber gloves.

Go for a spin until engine is warm.

Then remove the ht leads one at a time and see if there are any changes in engine idle speed.

I gave myself a shock when I tried this at the spark plug end.....

So I'd remove them all at distributor cap (with the engine off). Leave them over the distributor but not actually connected.

Put your rubber gloves on and get some long nose pliers.

Start the engine.

Lift cylinder ones ht lead off and listen for a change in idle speed. It should drop.

Put it back on then move onto the cylinder 2 and so on.

You may find that the engine idle speed doesn't change when you move one particular ht lead and then that might show you have an issue with a particular idle circuit.

Others will probably advise there are far better ways to do this, but I found this easier than removing the leads from the spark plug end!

It may show you nothing and may be more useful for finding blocked idle jets than the cause of your backfire.

Worth knowing though

 

There isn't much room above the carbs in my engine bay so I found long idle jet holders very useful.

Have you got a pdf of the dellorto book from CB performance it has some useful info. There jet sizing info may not be correct for Europe though.

I can email you it if you remind me of your address by pme

 

And your .3 emulsion tube is your biggest problem, you need .2's before you do anything else. The .1's plus adjusting the idle mix will cure all your problems. I'd sort them out if you weren't so far away.
I'd also go for the 28's so...
28 venturis
.2 emulsions
60 idle are fine for now
180 main airs are correct
Set the mains after you've done ALL that on a RR who should charge more like £100 or LESS - £400? they saw you coming.

 

This was Eurocarb's reply to my enquiry about vents & jet sizes for my 2110cc type one engine.
It may have some relevant info

Hope they don't mind. I've removed their personal info.

Thank you for all the info - usually we offer the below specs based on engine size but the choice of main venturi size will impact where you power is.



The smaller your main venturi (chokes in the above table) the better your low end torque and response but the lower your max RPM. Given what you've said it sounds like you would get better performance out of the 30mm setting instead of the 32mm.

The air correctors can probably stay at 175, they are close enough and only really impact higher end running - but the idle jet, main jet and chokes will be the important items to consider.

The jets need to match the choke size so I would advise changing the jets if you alter the choke size. The above specs are general and unfortunately can't account for engine modification but for approximately 95% of setups it should run well.

Kind Regards,

Eurocarb

 

Sorry yes .2 is the one, I've edited my post.
I have some 29 DRLA 36 vents here made from OG Dellorto 28's if you want them. No use to me - £20 posted?
Don't let Eurocarb tell you DRLA 40 vents fit - they do physically fit but create a step that messes with the air flow.

 

there must be another rolling rd that won't take your pants down!