Rejetting a 34 PICT-3 for 1776cc

After many months of work and countless delays and holdups, my newly built 1776 finally went for a short run last week. I now need to get everything tuned up which includes rejetting the carb which is currently way too lean. A few years ago I fitted a quiet pack exhaust system and rejetted the carb then to take account of that change. Now with the increase to 1776 I need to go larger again. I can't remember what jets I used last time but I'll have a look later in the week. Any advice on where to start? I don't want to waste too much money on trial-and-error testing to find the correct jets.

I ordered the jets from https://www.dellorto.co.uk/ last time but interested in any other suppliers.

 

Engine will be about 13% bigger than stock so a 127 (1.27mm or 1.61x10^-6 m^2) jet if going purely by area becomes 135 .

Alternatively if you already have a 130 jet then the step up is 137 or 140. (Should be 138 by area but the rate of flow is not really proportional to area, bigger holes flow faster than proportionally)

So try a 135 , 137 or 140, depending on where you started .. but the idle will probably stay the same because you can tweak the air and fuel on the idle circuit..

If this were in the States the recommendation would then be to hire an AFR meter and drive it..

 

Got mine from EuroCarb
Webers

 

every engine is different . you can`t just assume or guess .. AFR meter is the only true way to see whats going on

 

The need for measurement because its non- stock is why in days gone by I could have bought dual carbs for £500.. or a progressive carburettor for £250 and an AFR meter for £200, and a selection of jets for £50...


I bought the progressive and the AFR meter for the £500.

 

You could probably get away with the stock PICT jetting (1776 isn't that different), or just go up to a slightly larger main say 130. Other jets stay the same.

 

PS A 1776 will run reasonably well on the stock settings - you'll just miss out on a bit of power as you're not using extra fuel. The exhaust shouldn't really affect anything: you've un-bung one end, but unless you've got twin carbs, the top end is still restricted.

 

Reading the OP that it has been rejetted and is now determined to be running lean , that its probably already got a 130 main jet hence my comments.



But yes, if the carburettor is working in a proportional manner, as you suck air faster a 1776 is the same as running a 1568 engine at 13% higher RPM ..

It really shouldnt be getting very lean.. but the carburettor and stock manifold is an intentional flow restrictor, so eventually it is trying to starve the engine.


There is a 37mm PICT clone out there..Heritage sell them e.g,



Question: is the carburettor on this 1776 fitted under a stock aircleaner with hot a air feed?

...as running without enough heat is a cause for lean running on colder days.

 

@scrooge95 was running a 34pict 3 which came fitted on her 1776 before the Dells (??) .

If i was going down the 1776 route i`d happily fit a stock Solex with a 135 and keep an eye on the plugs , possibly re jet when there`s a good baseline to work from .

 

Found this article in the type2.com library. https://www.type2.com/library/fuel/jettype.htm
Yesterday, I sorted out some air leaks during my lunch break. Replaced the already cracking new rubber manifold boots and the metal gaskets for fibre ones, but ran out of time to do any testing. So I'm hoping I'll be OK with the jets I have. I think I have 130 main and 55 idle but will pull the carb tomorrow to check.

 

After 20 minutes driving, you may find you need to retorque the nuts - happens to me every time.

 

That'll double the amount of time it's been on the road in almost 18 months

 

This

I fitted a rebuilt Solex 34, with standard jetting to a 1776 a couple of weeks ago, and it ran beautifully throughout the revs. I dare say you could tweak it better with an AFR gauge, but for the usual van pottering about, standard jetting works fine.

 

Yup. I had my 1776 running on the stand with a stock Solex before fitting Dells, and it started and ran just fine.

You will be short of a bit of power, as you're not hosing extra fuel in to fully use the modest increase in cc-age.

 

Saturday I swapped out the 130 to a 135 main and attempted again to adjust the carb. Still popping slightly on the overrun but not as badly. The main problem is that I cannot get the idle speed down enough and the air screw is fully in. Unfortunately both my timing light and my DVM with iductive pick up are showing wildly fluctuating rpm readings, so I'm doing it by ear. It doesnt sound like its getting lower than about 1500 rpm. I've raised the carb and dealt with all the air leaks.

Sunday I went for a 50 mile shakedown run. A mixture of B roads, A Roads and some dual carriageway. All went pretty well. Good oil pressure and definately feels more lively. Just need to sort out this jetting issue.

 

If the air screw is fully in,
Your throttle plate is not closing properly, either through adjustment (throttle end stop screw should just be touching bottom step on the cold idle stepped cam when the choke is warmed up 100%.
Or the throat of the carburettor is worn and the plate is hanging up on the side of the throat on a wear step


Or the throttle spindle has ovalled out the hole in the carburettor body.


You might have a carburettor with a hole drilled in the throttle plate (originally done sometimes. ?? with the USA emissions DVDA ?? ) . Might have to solder it up.

You may have an air leak downstream of the carburettor. Listen around for sharp hissing noises.


Popping and banging can also be exhaust air leaks or ignition misfires, not just lean burning.

AFR meters are a lot cheaper than a set of cylinder heads or pistons..


Try moving the timing light / DVM pickup to different locations - maybe works better on the coil or right over near the number 1 sparkplug.. if you are using points, it could be a sign you could do with a new condenser, or the points are worn..

 

Try and get the pickup towards the plug away from the other HT leads.

 

Thanks Mike. Some good things for me to check there. I found my original Solex which I forgot I had rebuilt with new bushings and used that rather than the relatively new Kafer carb that I was originally using. So I've got carb options. Pretty confident I've found all the air leaks spraying Easy Start around the obvious areas, but will double-check.

I'm on a Powerspark system that's been working fine for me. Attached pickup at the #1 plug end and up at the dist without any difference. Could be me, could be the timing light. It's doing some odd things I don't remember it doing before.

I don't have any experience with AFR meters. Do they require a sensor mounting into the exhaust or use a probe like a gas analyzer? Is a DIY gas analyzer another option?

 

One thing I have seen, is that there is a big pulse from the coil when the current stops and the points open, and another smaller pulse that does not cause a spark when the current starts again as the points close. And the pickup on the timing light or DVM picks up the second pulse as well, so it counts twice as fast.

If you have a digital timing light and it is not getting a good RPM value, it is possible that it is also getting confused about timing.

On my DVM there is a slider on the pickup that lets you tweak the sensitivity but I always found it very fiddly to get a number off the DVM that matched the rev counter..

The old gas analyzers were a bit slow acting, they are really for setting up idle carbon monoxide percentages.
By the time you have calibrated them, and tried to take a reading of the exhaust gas, the hobby versions that turn up at car boot sales have drifted off zero by themselves.

Reading spark plugs is very retrospective and requires stopping pretty promptly after driving fast to look at the colour..

AFR lambda sensors come in the form of an M14 threaded sensor that is a bit like a spark plug in appearance but which has a flexible wire pigtail coming out of the end.

AFR meters can be fitted in a temporary fashion by sticking metal tubes up the end of the exhaust, with that sensor screwed into them, hoping the exhaust blows down the tube.

But as most bus exhausts have a bend just before the end, the sensor is too close to the outside air.

So you end up having to fit a "bung" to the exhaust somewhere, which is blanked off with an M14 threaded stop when no sensor is fitted.

The thing is that you need to get a measurement to check for lean running at full power and also when just backed off full throttle or even rolling fast on a slight downhill
The mixture can change a lot if you are trying to keep it down to 55mph and the wide open throttle of your engine wants to take you past 70mph .. and that "55mph" mixture can be lean and relatively hot burning, while the wide open AFR is 12.5:1 , giving the strange effect that it runs noticeably hotter if you try to treat it nicely.

And for that an AFR meter gives you updates second by second of whether the mixture is lean or rich.

A rolling road testing feeding energy into accelerating a rotating mass can only really set up the mixture for acceleration, great for the traffic lights and the drag strip. It cannot very easily measure steady state mixture representing cruising on a flat road or climbing a slight hill . For those you need the more complex brake based dynamometers, where you can set the engine load and then measure the engine behaviour.

 

I'll need to do more reading on this before deciding to try this. From what I've read today the sensor needs to be at least 18"-24" away from the engine to avoid long-term damage. You'd need a long cable to wire it to a dashboard gauge. Is that why your method is a temporary diagnostic setup? Where would you fit the sensor on a quiet pack exhaust like this? Near the collector? Also, what's the difference between wideband and narrowband? Is one more appropriate than the other? I've got a lot to learn about this subject.

I have a Gunson Colortune that I've used for decades on my other classics but I have never tried it on an aircooled engine mainly because of access and viewing the plug. It might be worth a go.