I have a 1970 MGB Roadster and have a running problem
Things I have done to the MG in order to try and get it to run right. First of all, problem is this:
Car starts and idles perfectly. Car will not rev higher than 3000rpm. If you try to hold revs at 3000rpm the revs die off after about 3 seconds and car starts to cough and splut (does not go as far as backfiring) but car will rev down and almost cut out until you release throttle and car recovers and idles perfectly again.
Things I have done:
Ignition
1. Fitted electronic ignition (replaces points and condenser). Before this, I fitted new points and condenser - no change.
2. New coil.
3. New dist cap.
4. New rotor.
5. Tried different set of leads (borrowed from TR6 - no difference - so assume leads ok).
6. Checked vacuum advance is working by sucking on tube at carb.
7. Checked advance springs are intact in dist (removed base plate and looked inside - all there correct and clean).
8. Set timing using strobe light (16 deg BTDC in this case) but also tried lots of other positions and test drives.
9. Checked TDC marks on pulley and case are correct by moving piston 1 to TDC, checking rotor pointing to No.1 lead and checking timing marks aligned - all ok.
Fuel
1. Removed fuel pump. Cleaned (was not really dirty), checked points - cleaned points (slightly pitted), pump run on bench. Looks and sounds ok. No filters in pump.
2. Attached clear tube after filter before carbs and run pump into container - plenty of fuel coming through although quite alot of air bubbles mixed in.
3. Removed pots and pistons from carbs and cleaned all inside.
4. Removed fuel pots, floats and needles - cleaned - all ok.
5. Set mixture using colortune kit - set it slightly rich to be sure.
6. Checked throttle flaps opening together and not sticking.
7. Checked choke not stuck on.
8. Set correct idle.
9. Shut off fuel pump and replaced with elevated gravity feed from bottle (held out of window by Elaine along main road!). No difference.
Engine
1. Checked gap on tappets - set all slightly wider than spec to make sure nothing too tight - bit rattly now but ok.
2. Oil all ok.
3. Water all ok.
4. No smoke or vapour from exhaust.
I would be grateful for any other ideas you might have. It's killing me.
One last thing, when it first went wrong, pulling the choke out at 3000rpm did allow it to climb above 3000rpm and improved the performance, so does this suggest a fuel problem? You would think so (as I did) but having checked all fuel things, not sure what to think now. Please help.
Cheers for now. Elaine
Edited by Dynamic Dave on 17/07/2009 at 15:41
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sounds like a weak pump to me
are they dear? off say rimmer brothers? if not i would change it to rule it out seeing as you havent got facilities to check the fuel pressure rate
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Quite possibly. The clue I think are the bubbles in the fuel, when the engine is under load it is demanding much more fuel which it appears is not being supplied.
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As a starter, stick the colourtune on and see what happens at 3000 rpm, if it goes weak then you can rule out all the ignition side of things, if it stays the correct colour (blue IIRC) then you can rule out the fuel side.
BTW you don't mention changing the spark plugs, have you ?
Have you tried the colourtune on all 4 cylinders ?
Also try fresh fuel and double check the firing order, I came across a b-series engine in a marina that would run ok on tickover, but not rev when the leads were mixed up.
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Hi Elaine,
I know this is a trivial thing but did you fill the Carb dashpots with oil after reassembly. These engines won't rev if the dashpots are empty.
Regards
Robin the Technician
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Intermittent internal collapse of flexible fuel line?
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Float levels are critical & if set wrong can cause this but has the exhaust been checked for partiall blockage.
As the revs increase it causes too much back pressure making it run weaker & weaker till it dies out.
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From a very good source, I am told this is a blocked exhaust. Check it out.
I can say no more as I am only passing on someone's opinion.
Rob (Moderator)
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ihave had same problem on a tr6 (tr6 import running su carbs) had so much trouble same as you spoke to specialist an he advised to replace the neadles in the carbs, changed them and what a difference, it would not pull above three grand without pulling the choke out (proves its a fuel mixture not a supply problem) however the bubbles do concern me,Regards TB
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There should be no bubbles in the fuel, this implies the pump is sucking in air between the tank and the pump, however if you really have plenty of flow then this may be only part of your problem a block exhaust would explain the dying as when the back pressure builds it leave combustion gases in the cylinders and this bleed back out into the inlet manifold when the inlet valves open and prevents the carbs from fuelling. Regards Peter
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Change the plugs, their dielectric breaks down as they get old, however I suspect that the exhaust, fuel pump or needles are the real issue.
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Hey guys
thanks so much for all the suggestions we used some of them and managed to get a new fuel pump today and it is running fine now...............thanks again to all of you its great to know there is so much knowledge out there.
Regards
Elaine
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