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Normandy Jim

GMC engine trouble...

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First post for a while, got a GMC running Like a bag of nails, new headgasket fitted, four new push-rods, valves and springs on no 1+2 cylinders, carb + fuel pump overhauled, LT and HT system changed With new coil..enging timing set up and strobed, fins well fire 10-15 mins then starts to miss on 1+2 again, got 9.5 bar compression through all 6 cylinders, good sparks, checked, rechecked, valve clearances, CBs, etc, all good...

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Assuming you are certain the problem lies with nos 1 & 2 cylinders, and bearing in mind that you have replaced most of the valve train for these two, have you checked that the valves and rockers are receiving oil - if not, the valves could stick as the engine warms up, and the problem will disappear once it has cooled again. Presume that you also replaced the spark plugs? Try replacing them again, even new ones can sometimes be faulty.

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Posted (edited)

Fire it up, wait till it starts acting up, then shoot some WD40 at the inlet manifold to head connection for 1 and 2.  

I'm thinking it might be moving when it heats up and starts drawing air.

Edited by Gordon_M

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Curious ...do you have fresh fuel .....? I seem to regularly run into issues with poor fuel..I often start engines for the first time with high octane fuel...? 

Jenkinov

 

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my question would be why in the first place did you need to replace the valve gear on just . only on those two cylinders ?

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On 4/7/2020 at 12:12 AM, mike30841 said:

Assuming you are certain the problem lies with nos 1 & 2 cylinders, and bearing in mind that you have replaced most of the valve train for these two, have you checked that the valves and rockers are receiving oil - if not, the valves could stick as the engine warms up, and the problem will disappear once it has cooled again. Presume that you also replaced the spark plugs? Try replacing them again, even new ones can sometimes be faulty.

All ignition system is new, problem lies with 1+2 as when HT leads are disconnected the engine tone stays the same, disconnect the other 3<6cyl  engine tone drops..Im running engine without rocker cover and side covers, getting oil to all..Will be back on it again on Monday...Will let you know the outcome..

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11 hours ago, wally dugan said:

my question would be why in the first place did you need to replace the valve gear on just . only on those two cylinders ?

The pushrods on 1+2 cylinders were damaged, be that being due to sticking cam followers or valves or weak return springs????  so to be safe all was changed..

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20 hours ago, jenkinov said:

Curious ...do you have fresh fuel .....? I seem to regularly run into issues with poor fuel..I often start engines for the first time with high octane fuel...? 

Jenkinov

 

The fuel is fresh and new, with additive and filtered, besides that would effect all cylinders not just 1+2...But thanks anyway

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7 minutes ago, Normandy Jim said:

All ignition system is new, problem lies with 1+2 as when HT leads are disconnected the engine tone stays the same, disconnect the other 3<6cyl  engine tone drops..Im running engine without rocker cover and side covers, getting oil to all..Will be back on it again on Monday...Will let you know the outcome..

Take out no's 1 and 2 plugs and swap them with 5 and 6, see if the problem moves. Strange that before the repairs, cylinders 1 and 2 were giving trouble. As has already been mentioned, it could be drawinng air on the intake manifold joint which opens up when the engine gets hot. These exhaust manifolds are prone to warping and if machined then the intake manifold has to be carefully matched to it so all the faces are in line.

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22 hours ago, Gordon_M said:

Fire it up, wait till it starts acting up, then shoot some WD40 at the inlet manifold to head connection for 1 and 2.  

I'm thinking it might be moving when it heats up and starts drawing air.

Will try that on Monday, when i"m next at work, Had the manifold skimmed, new central locating rings and gaskets fitted...Shall give you the news next week...thanks

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7 minutes ago, Richard Farrant said:

Take out no's 1 and 2 plugs and swap them with 5 and 6, see if the problem moves. Strange that before the repairs, cylinders 1 and 2 were giving trouble. As has already been mentioned, it could be drawinng air on the intake manifold joint which opens up when the engine gets hot. These exhaust manifolds are prone to warping and if machined then the intake manifold has to be carefully matched to it so all the faces are in line.

Thanks Richard, It's the first time thats happened to me, shall check it over Monday..

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Just now, Normandy Jim said:

Thanks Richard, It's the first time thats happened to me, shall check it over Monday..

Jim,

Just seen your previous post about skimming manifold and fitting new rings. If you skimmed both manifolds together then the rings may be a bit too wide now and not allowing the manifold to compress the gasket. I normally measure the rebate in the head and manifold, the width of ring and thickness of gasket to ensure the assembly will fit correctly with no pressure on the rings.

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On 4/8/2020 at 10:18 PM, Richard Farrant said:

Jim,

Just seen your previous post about skimming manifold and fitting new rings. If you skimmed both manifolds together then the rings may be a bit too wide now and not allowing the manifold to compress the gasket. I normally measure the rebate in the head and manifold, the width of ring and thickness of gasket to ensure the assembly will fit correctly with no pressure on the rings.

Good morning Richard,

Bad news on Engine, started and ran well for an hour, everything normal, temperature stayed 140..Then started running erratically, overheating and serious vibrations...back to square one again..fun fun fun..Thankfully we have no clients visiting at the moment so no tours(although no clients no money coming in) so time to solve the problems..After this got the 1937 SAURER, Bedford QL, and Humber...2 big projects rebuild of QUAD and MATIDOR...Thanks for your help will keep in touch, keep safe in these bizarre times.. 

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On 7 April 2020 at 11:20 PM, Gordon_M said:

Fire it up, wait till it starts acting up, then shoot some WD40 at the inlet manifold to head connection for 1 and 2.  

I'm thinking it might be moving when it heats up and starts drawing air.

 

On 7 April 2020 at 11:20 PM, Gordon_M said:

Fire it up, wait till it starts acting up, then shoot some WD40 at the inlet manifold to head connection for 1 and 2.  

I'm thinking it might be moving when it heats up and starts drawing air.

Hi Gordon, sorry not responded, got the old girl running, there was a problème With the manifold, when it was skimmed they she ares. 2 off the bolts, repaired it   But caused an interna crack between the joints..removed and stripped down then repaired...bingo..Thanks for your assistance anyway,,take care

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As long as it works Jim.  If you get the chance of a spare manifold, grab it and keep it one the shelf.

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33 minutes ago, Gordon_M said:

As long as it works Jim.  If you get the chance of a spare manifold, grab it and keep it one the shelf.

Got one in stock, and another to be skimmed, got 3 GMC engines for rebuild in the future as well...Soon to start on the ROYAL DUKW...

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Got a full set of DUKW stencils in the garage from 14962, and a Parts List  and so on.  Don't think I have many mechanical bits though.  Good luck with it.

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