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Bedford OY 28hp lacking power


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Hi All,  I have a Bedford Oy which had a badly blowing exhaust manifold gasket. Changed the gasket but now the engine lacks low down power, its gutless until its on max revs.  Ran perfect before the gasket change.  Starts and ticks over fine, its a bit lumpy on acceleration though.  Checked the vacuum advance on the carb and there is a weak vacuum, only enough to hold a postage stamp in place when on light to moderate revs.  I'm wondering if that's the problem, a lack of sufficient vacuum to pull the advance rod at the distributor?  I have checked the pipework and the diaphragm on the distribultor which seem ok.  When I had the manifolds/carb off to do the gasket could I have affected something in the carb?  Any and all thoughts welcome!!  Thanks, Jim.

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Hi Jim

are you using standard gaskets or the copper sandwich ones available from Norman Aish?

If the standard ones, are you sure that they are orientated the correct way as apparently you can put them on the wrong way - leading to a leaky join between the head and manifold.

 

simon

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Often the manifold gaskets are blamed for these sort of problems, but it is by no means uncommon to have a bowed exhaust manifold. These can be checked with a straight edge and need to be within .006"  (6 thousands of an inch). A specailist engine machinist can correct this in a lot of cases and once assembling the inlet manifold back on to the exhaust you need to ensure both manifolds are in alignment. A bit of care taken here pays off, and only use the copper sandwich gaskets as they have more compression ability to take up minor errors.

I have done countless of these manifolds and once correct will be good for a long time.

regards, Richard

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56 minutes ago, simon king said:

Hi Jim

are you using standard gaskets or the copper sandwich ones available from Norman Aish?

If the standard ones, are you sure that they are orientated the correct way as apparently you can put them on the wrong way - leading to a leaky join between the head and manifold.

 

simon

Hi Simon, hope your well.  Yes, used the thick ones from Norman. Jim

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32 minutes ago, Richard Farrant said:

Often the manifold gaskets are blamed for these sort of problems, but it is by no means uncommon to have a bowed exhaust manifold. These can be checked with a straight edge and need to be within .006"  (6 thousands of an inch). A specailist engine machinist can correct this in a lot of cases and once assembling the inlet manifold back on to the exhaust you need to ensure both manifolds are in alignment. A bit of care taken here pays off, and only use the copper sandwich gaskets as they have more compression ability to take up minor errors.

I have done countless of these manifolds and once correct will be good for a long time.

regards, Richard

Hi Richard.  Yes they are great gaskets.  Checked with  straight edge all faces when removed and all fine.  Its certainly sealing well, I have a great brake pedal, manifold vacuum A1.  I'm sure its nothing to do with the gaskets, but maybe the carb.  I left the carb on the manifold when removed.  The port vacuum (suck) coming from the carb is non existent on idle, then, on low to moderate revs I get a very light vacuum, doubt it would operate the advance plate in the distributor.  Would the vacuum be greater when the engine is under low down load?  My understanding is that when the throttle plate is opened, vacuum from the intake manifold is transferred to the port in the bottom of the carb which then goes to the distributor and advances the spark, or does it, I'm still learning?  Also, could it possibly be the cut off flap thingy in the exhaust manifold box? TIA, Jim.

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13 minutes ago, jim fl4 said:

Hi Richard.  Yes they are great gaskets.  Checked with  straight edge all faces when removed and all fine.  Its certainly sealing well, I have a great brake pedal, manifold vacuum A1.  I'm sure its nothing to do with the gaskets, but maybe the carb.  I left the carb on the manifold when removed.  The port vacuum (suck) coming from the carb is non existent on idle, then, on low to moderate revs I get a very light vacuum, doubt it would operate the advance plate in the distributor.  Would the vacuum be greater when the engine is under low down load?  My understanding is that when the throttle plate is opened, vacuum from the intake manifold is transferred to the port in the bottom of the carb which then goes to the distributor and advances the spark, or does it, I'm still learning?  Also, could it possibly be the cut off flap thingy in the exhaust manifold box? TIA, Jim.

Hi Jim,

All sounds good re manifold condition. However if the exhaust heat flap is not working and stuck in the starting position there will be too much heat building up in the intake and could cause running problems.

Check your vacuum fittings to wipers, if it has vacuum wipers, as there may be a loss of vacuum due to a leaking fitting or pipe.

Richard

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8 minutes ago, Richard Farrant said:

Hi Jim,

All sounds good re manifold condition. However if the exhaust heat flap is not working and stuck in the starting position there will be too much heat building up in the intake and could cause running problems.

Check your vacuum fittings to wipers, if it has vacuum wipers, as there may be a loss of vacuum due to a leaking fitting or pipe.

Richard

Hi Richard,

I will check the exhaust flap tomorrow. No vacuum wipers, definitely a good healthy intake manifold vacuum, as proven by a nice steady idle and the brake servo stopping her great. Starts great and the cold start can be pushed fully home after 15- 30 seconds or so.  Carb mixture screw 1 and 1 quarter turns out.  Do you think the very light vacuum coming from the carb port is normal when on low to moderate throttle? Thanks, Jim.

 

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It would be worth checking the rubber hose connections to your wipers and/ or advance retard diaphragm first for leaks or splits also the joint between carb and inlet manifold. 

 If they check out it would be worth investing in a a vacuum gauge ( the  non damped version from the usual sources on e bay or your local car factors) don't spend huge sums of money £10 to £20 max should get you what  you need,  you are looking for trends not definitive numbers.  Temporarily  connect the gauge to your vacuum wiper port  or any other suitable location below the throttle plate on the carb or inlet manifold.

Run some diagnostic checks (google how to read a vacuum gauge if you have not used one before).  This can all be done in the workshop no need to take the truck out on the road.

Depending on what you find with the vacuum gauge the next test would be with a compression gauge on each cylinder again google how to do this if you have not done it before results need to be done first dry then with a small amount of oil down the plug hole.

In both cases check the results with the diagnostic charts,  it could be one of a number of  issues or a combination of them the combined results from vacuum and compression will point you in the right direction.

A little time spent now while the engine is complete and running will save you endless hours of grief and quite possible 100's of £'s wasted on doing unnecessary work or going round in circles.

Pete

Edited by Pete Ashby
missing couple of words
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2 hours ago, Pete Ashby said:

It would be worth checking the rubber hose connections to your wipers and/ or advance retard diaphragm first for leaks or splits also the joint between carb and inlet manifold. 

 If they check out it would be worth investing in a a vacuum gauge ( the  non damped version from the usual sources on e bay or your local car factors) don't spend huge sums of money £10 to £20 max should get you what  you need,  you are looking for trends not definitive numbers.  Temporarily  connect the gauge to your vacuum wiper port  or any other suitable location below the throttle plate on the carb or inlet manifold.

Run some diagnostic checks (google how to read a vacuum gauge if you have not used one before).  This can all be done in the workshop no need to take the truck out on the road.

Depending on what you find with the vacuum gauge the next test would be with a compression gauge on each cylinder again google how to do this if you have not done it before results need to be done first dry then with a small amount of oil down the plug hole.

In both cases check the results with the diagnostic charts,  it could be one of a number of  issues or a combination of them the combined results from vacuum and compression will point you in the right direction.

A little time spent now while the engine is complete and running will save you endless hours of grief and quite possible 100's of £'s wasted on doing unnecessary work or going round in circles.

Pete

Thanks Pete, I will get a vacuum gauge and do the test you've mentioned.  I'm confident its a minor thing, because the vehicle was running perfect, (save for a blowing manifold) until I removed the manifolds complete with carb. So, one would think its something to do with those parts and the reassembly thereof.  Thanks again, Jim.

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Your welcome Jim, I'm basically lazy so I do all the simple stuff first.

Your problem particularity as you say it ran fine before you changed to gaskets suggests a fairly large air leak on the induction side of your engine that's why your not seeing much in the way of vacuum.   Vacuum = absence of air or something lower that atmospheric pressure  if you like,  so to be pedantic your problem is not lack of vacuum it's too much air leading to low or no vacuum it's perhaps easier to think of it this way round so check the hoses and connections first. 

Then with the engine running spray a little WD40 around the base of the carb/inlet manifold joint if the engine speeds up the leak is there and then do the same on each inlet flange/to block joint.

The vac gauge, compression gauge and a cheap multi meter to measure dwell and engine RPM are your best and most used tools on these old engines using them you can diagnose mechanical problems, set timing, set dwell and fault find your ignition and lighting circuits. 

Good luck and come back and tell us what the problem was it's always useful for all of us to hear the result.

Pete

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6 hours ago, Pete Ashby said:

Your welcome Jim, I'm basically lazy so I do all the simple stuff first.

Your problem particularity as you say it ran fine before you changed to gaskets suggests a fairly large air leak on the induction side of your engine that's why your not seeing much in the way of vacuum.   Vacuum = absence of air or something lower that atmospheric pressure  if you like,  so to be pedantic your problem is not lack of vacuum it's too much air leading to low or no vacuum it's perhaps easier to think of it this way round so check the hoses and connections first. 

Then with the engine running spray a little WD40 around the base of the carb/inlet manifold joint if the engine speeds up the leak is there and then do the same on each inlet flange/to block joint.

The vac gauge, compression gauge and a cheap multi meter to measure dwell and engine RPM are your best and most used tools on these old engines using them you can diagnose mechanical problems, set timing, set dwell and fault find your ignition and lighting circuits. 

Good luck and come back and tell us what the problem was it's always useful for all of us to hear the result.

Pete

Hi Pete,  Thanks for your advise.

I agree with you that it would sound to be leaking air into the intake manifold. However, surely if this was the case it would not start and idle exactly as before, no mixture adjustment or use of choke either, also the brake servo has tons of vacuum when I touch the brake pedal!  Very puzzling but whatever it ends up being I think it must be on or near the manifolds.

Thanks again, Jim.

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Now then everyone.  Problem solved.  It was, after all just a case of a leaking union on the capillary vacuum pipe from the carb to the dizzy.  These engines just will not run correctly if there is a leak on this pipe.  Thanks once again to all for their help in 'brain  storming', great guys indeed. Jim.

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