mgb choking on hard acceleration dellorto dhla45

[muso58]

Hi Guys . old school mechanic of some 45yrs . totally confused .
B series engine , chokes when i floor throttle. swapped plugs ,leads, distributor , took head off checked and re fitted , tuned delortto carb to best of my ability . Dont think its a a carb problem as i also get this symptom .
pull off no 1 plug on tickover no noticable difference, pull no4. plug off no noticable difference if i do this with 2 and 3 it dies (as it should). if it was a carb problem surely the problem would be confined to one bank back or front! as it is in effect 2 seperate carbs in one . compressions were checked 175psi across all cylinders . it has loads of power when off tickover driving it.
Am i missing something here ? cheers for looking

[stevieturbo]

instead of randomly throwing parts at it, test.

Do you have a good confirmed spark at all cylinders ?

You say it is a B series, this is a siamese port head. So with your carb, is it divided 1/2 and 3/4, or other ?

When did it last run correctly ? and what has changed since then ?

you seem to say at idle, low loads etc it runs correctly, drives well, except you contradict this by saying there is no change when you remove 2 of the plug leads, implying those cylinders are not firing.
So do those plugs have good spark at that time ? Are those plugs clean, wet, dry, other ?
Does the temperature of each exhaust tube suggest they are not firing ?
Can those exhaust tubes be blocked ?

What are fuel mixtures like when driving ?, when you stand on the accelerator is it going lean or rich ? Is fuel supply to the carb good ? what is fuel pressure like ?

[GreenV8S]

What's the vacuum advance system like on that engine? If it uses ported vacuum with a blocked / wrongly installed delay valve you might find the timing is doing weird things when you come off idle. If present, then as a sanity check you could just disconnect and block off the vac line temporarily so that you run on the static timing.

[Equus]

muso58 said:

... if it was a carb problem surely the problem would be confined to one bank back or front as it is in effect 2 seperate carbs in one

It only has one accelerator pump, and the symptoms you're describing (if I understand them correctly - your written English isn't the best) would seem to correspond to what you'd expect from a faulty pump. Have you checked the diaphragm on the accelerator pump, as a start? Then check the stroke of the pump (which can be adjusted).

[stevieturbo]

So I'll repeat again...to ensure it is not missed.

When did it last run correctly, if ever ?

And add a response from a user more experienced than a lot of us with carbs, but is struggling with passwords to get back online, in part PH being awkward. ( Peter Burgess )

"The guy is suffering from charge robbing associated with siamese ports and the curved manifold with a sidedraft carb. I get loads of people phoning with problems with 1 and 4 but perfect compression, when I ask if carb is Weber/ Dellorto sidedraft they say how do you know!!!!!"

[muso58]

stevieturbo said:

instead of randomly throwing parts at it, test.

Do you have a good confirmed spark at all cylinders ?

You say it is a B series, this is a siamese port head. So with your carb, is it divided 1/2 and 3/4, or other ?

When did it last run correctly ? and what has changed since then ?

you seem to say at idle, low loads etc it runs correctly, drives well, except you contradict this by saying there is no change when you remove 2 of the plug leads, implying those cylinders are not firing.
So do those plugs have good spark at that time ? Are those plugs clean, wet, dry, other ?
Does the temperature of each exhaust tube suggest they are not firing ?
Can those exhaust tubes be blocked ?

What are fuel mixtures like when driving ?, when you stand on the accelerator is it going lean or rich ? Is fuel supply to the carb good ? what is fuel pressure like ?

Yes good spark at all cylinders .ive even moved dissy thru 90 deg and repositioned leads problem stays with 1 and 4 .
carb front bank feeds 1 and 2 rear 3 and 4
i bought it like this so dont know when it last ran correctly
at idle at standstill engine doesnt seem lumpy.
i realise it sounds contradictory ,to say it seems smooth on tickover , but removing 1 or 4 seems to make little difference while ticking over . if i remove 3 or 4 it does as you would expect.!
if i take revs up to say 1500 , by operating throttle lever, then pull 1 or 4 it runs lumpy as you would expect .
ive swapped over coil , removed electronic ignition and replaced with old style points no change .Ive swapped leads no change.Ive swapped plugs no change . all compressions are atound 150psi.
I removed exhaust and ran without no change .I road tested it and all plugs had nice light brown colour ..

.

[muso58]

[quote=GreenV8S]What's the vacuum advance system like on that engine? If it uses ported vacuum with a blocked / wrongly installed delay valve you might find the timing is doing weird things when you come off idle. If present, then as a sanity check you could just disconnect and block off the vac line temporarily so that you run on the static timing.[/quote

Hi it has adv retard within lucas dissy on this type . no tube to manifold

Edited by muso58 on Sunday 25th September 15:43

[muso58]

Equus said:

It only has one accelerator pump, and the symptoms you're describing (if I understand them correctly - your written English isn't the best) would seem to correspond to what you'd expect from a faulty pump. Have you checked the diaphragm on the accelerator pump, as a start? Then check the stroke of the pump (which can be adjusted).

Has new diaphragm by the look of it . Yes i removed pump assembly as i was erring that way. removed jets no apparant problem . but why would accelerator pump cause no apparant change on tickover when i pull 1 or 4 ?

Edited by muso58 on Sunday 25th September 11:54

[muso58]

stevieturbo said:

So I'll repeat again...to ensure it is not missed.

When did it last run correctly, if ever ?

And add a response from a user more experienced than a lot of us with carbs, but is struggling with passwords to get back online, in part PH being awkward. ( Peter Burgess )

"The guy is suffering from charge robbing associated with siamese ports and the curved manifold with a sidedraft carb. I get loads of people phoning with problems with 1 and 4 but perfect compression, when I ask if carb is Weber/ Dellorto sidedraft they say how do you know!!!!!"

I bought it with this issue . its a dellorto dhla 45 side draft . "charge robbing with siamense ports " sounds interesting ?

[stevieturbo]

muso58 said:

I bought it with this issue . its a dellorto dhla 45 side draft . "charge robbing with siamense ports " sounds interesting ?

It is a known issue when people try to fuel inject such engines.

I've never heard of it before with carbs, but Peter did specifically say it was due to the curved intake port with that carb setup.


http://www.peter-burgess.com/

[GreenV8S]

muso58 said:

i realise it sounds contradictory ,to say it seems smooth on tickover , but removing 1 or 4 seems to make little difference while ticking over . if i remove 3 or 4 it does as you would expect.!

Pay attention to stevieturbo's comment about exhaust header temperatures. This will tell you definitively whether each cylinder is actually firing. If you've never seen the engine running cleanly then don't assume you will be able to judge whether it's misfiring.

[stevieturbo]

And equally pay attention to what Peter asked me to say about the known carb issues with that setup.

I presume the fix is change it.

[muso58]

Hi guys just to keep you all in the loop.
I spoke to eurocarb today .very helpful.
I had already sussed that jet's were meant for the old 2 litre osseti block from a past era!
40mm venturi was far to big .as we're other jets .
Travin confirmed .without me prompting him.
New jets and ventures on way,
designed for this engine. Only
£70 incl del .
Will let you know what happens . Still may end up putting SUs back on !

[muso58]

Jets changed no difference . Ive now pulled pistons out to check bores etc . they look less than 1000miles old !
One thing i have found is either 3 rods are round wrong way or one is !
ie one has oil hole facing opposite direction to the rest !!
Not relevant i suppose but needs sorting before i put pistons back in .
At least i know block isnt the problem .
Im going to rebuild it after having head pressure tested and valves re done .
If no change SUs are going on it .

[stevieturbo]

An oil hole ???

and you don't seem to be listening about the carb/intake.

Pulling the engine apart seems a little drastic, although if something is assembled wrong...I guess may be worth it. But will not be your carb problem

[muso58]

stevieturbo said:

An oil hole ???

and you don't seem to be listening about the carb/intake.

Pulling the engine apart seems a little drastic, although if something is assembled wrong...I guess may be worth it. But will not be your carb problem

Yes i listen to everything . I have time on my hands and like to know the condition of everything anyway.
It only took about 1 hr to drop the sump and pop the pistons out as head was already off.
I enjoy messing with engines ,which im guessing a lot of folk on here do .
.I realise this is decidedly looking like charge robbing . But at least if i look at everything else i will be more sure.
Cheaper to pull my motor to bits than buying a new SU carb/manifold .
Ive since been told by a local engine repair outfit that A series engines, can have con rods either way round .May be the same with B series i dont know yet
Its obviously a case of someone not paying attention when they were mounting the pistons.

[stevieturbo]

A- series has offset rods. They cannot go either way round. Do not listen to whoever told you that

I cannot comment on B series as never had one apart.