Revving problems

[AlanH]

In an attempt to sort out my rough running engine I have given the SUs an overhaul.

New jets (changed from waxstats), needles and float chamber valves plus a good clean inside and out. I also blew out the fuel line.

It ticks over OK and revs freely, with just the occasional slight miss, when it is standing still. However when driving it accelerates very slowly after about 2000 revs and will not go above 3000.

Anybody got any ideas/


There is another problem, I'm not sure if it's got anything to do with the one above. The rear carb bowl overflows when the choke is on. I have checked the valve and it's fine. The problem goes away when the choke is pushed in.

[Jon Tilson]

Check the clamps are tight on the rod between the throttles. You may well just be pulling only one carb open.

If its flooding your float valve is not working. Take it right out of the bowl lid and blow it through.

Jonners

[Dolly-Nut]

Have you reset the jet height/mixture?

[trackerjack]

Float bowl needle jets are unreliable and can often have a little gunge in them that is hidden but sometimes stops the needle from shutting off the fuel.
If there seems to be no trouble with gunge but you stilll find flooding try smearing some metal polish like Autosol on the pointed bit and swivelling it on its seat.

Failing that pay the horrendous prices asked for new ones and change em.

[dalbuie]

The problem on my 1500 FWD was quite similar and I traced it to an electrical problem. The main feed - on a FWD - goes down to the solenoid and the connectors down there were a bit loose, the cables were very hard being so close the the exhaust. I had stripped the single carb looking for issues there. Obviously enough power to tick over and rev a bit without load but as soon as it was on the road there wasn't enough. Might be worth looking at.

[AlanH]

Thanks for the replys.

Being as it's breaking down under load i'm tending towards electrics.

Will have a look and report back.

[trackerjack]

Our old MG Montego would misfire and refuse to rev as did my old 1500 HL Dolly and the fault was the coil is mounted on the inner wing and would arc onto the body, on both these cars and the Sprint never suffers as its in a warm bulkhead position.
Run it in the dark to see it too.

[xvivalve]

What condition are your points in?

[AlanH]

What condition are your points in?

New points, new capacitor, new distributor cap, new coil, new plug leads, new plugs.

I have put the timing strobe on each of the leads and they are firing.

However, setting the points gap is a devil of a job with an automatic so will revisit that and also check the capacitor. I bought some advance/retard springs from Robsport and they seem a bit weak. I'm definitely thinking of changing to a 45d distributor.

Unfortunately my Herald has also thrown a strop with what could well be a nasty problem (it's more than likely upset at the lack of attention !), so I have quite a lot on.

[AlanH]

After a lot of poking and head scratching I decided in desperation to change the carbs (again).

A friend of mine has an old pair which I cleaned up and stuck on. The revving problem immediately went away but the mixture is always rich even with the jets screwed right in. I'm not too worried about this at the moment, it could just be worn jets. However, I really am at a loss to figure out what else I can do. I thought that it may be a sticking piston so I put in the dashpots and pistons from the original ones but that made no difference. I think that the only thing left is the butterfly valves.

Another worrying thing is that I still am getting petrol spewing out of the rear carb bowl. I have changed the float, needle and seat twice including new ones and ones that I am sure are OK.

I have had a thought however. I recently managed to purchase a set of alloy carb mounts on ebay which do not flex. Given that the engine is running a bit rough I wonder if the shaking is causing the float to slop about and the needle to open up.

[Jon Tilson]

As we said before if your carb float bowl is overflowing then you have a disfunctional float valve.

It really shouldn't be that hard to solve. Float level has a very strong relationship to the mixture. If its too high it will be too rich. Any SU with an overflow problem will run rich.

You have to sort it. As it seems common to another set of carbs I have to ask if you have something odd
in the fuel pump department. Does it have an electric pump? Or a non standard mechanical one?

Too high a fuel pressure can overcome the float valve.

Jonners

[Toledo Man]

The 45D4 dizzy won't fit the 1850 but you can do what I did and fit the 44D4 dizzy from the Sprint. You can keep the existing HT leads and you can use the points, condenser, rotor arm and dizzy cap from a 45D4 dizzy (as used on the 1300 & 1500 engines). This will make your life much easier even if you decide to stick with points and gives you more options for electronic ignition should you wish to go down that route.

Why do I always miss out on alloy carb mounts? I could do with a pair for mine. I'm prepared to pay for a set from the Club when the time comes. I really need to sort out that subframe mount first.

[red&black]

[quote Another worrying thing is that I still am getting petrol spewing out of the rear carb bowl. I have changed the float, needle and seat twice including new ones and ones that I am sure are OK.

.[/quote]
Is the fuel tank breathing properly? A faulty filler cap or blocked breather will pressurize the system and can cause this kind of problem.
If so, you will probably get a release of pressure when removing the filler cap.

James

[Jon Tilson]

I dont see any mechanism for pressurising a Dolomite fuel tank. Only sucking petrol out if the vent is
blocked will cause a loss of pressure, not an excess.

Jonners

[Toledo Man]

If an aftermarket fuel pump has been fitted it could be creating excess pressure resulting in overflowing carbs. A regulator will get round this problem.