GMC sluggish to achieve 45MPH !!!!!!!!!!!!!!

[R Cubed]

I am stupid, well so many friends say, but am I getting parranoid, my GMC 352 seems to be sluggish to get to 45MPH can any one give me some idear's of how long it takes to get to 45MPH from a stand still but under normal acceleration and flat roads, or say from 20 to 45, it just seems the throttle pedal is on the floor all the time to get it up there eventually, I dont know if the engine is down on power or is it that I have not driven it for such a long time.. Just a warning dont use the truck speedo mine is 5 to 10 MPH slower than Tom Tom says :dunno:

 

Feedback eagerly awaited

 

R Cubed

[Richard Farrant]

You should initially check tyres pressures are correct, and that none of the brakes are binding.

[Snapper]

sluggish ? - try my Iltis.

[LeeEnfield]

sluggish ? - try my Iltis.

 

 

 

Or my MW.............. :whistle:

[MARTIN CROSS]

Hi

Has your front axle become engaged? Jack up a wheel to make sure it spins freely. I recall my DUKW (and therefore presumblyt a Jimmy ) had a tap to isolate the front brake hydraulics. Could this be at fault causing the brakes to stick on? Also, is the fuel filter blocked?

Good luck.

M

[R Cubed]

Ok thanks for all the replies, no brake bind, front axle not engaged, tap removed when I replaced all lines ( two less joints to leak !! ) tyre pressure a bit low on fronts but 45psi is what it has always been. Must be me but would be nice to here from other Jimmy owners of how their trucks perform..

 

R Cubed

[TooTallMike]

My old one accelerated like a scalded cat.

 

Have you looked at possible vacuum leaks on the intake side? Easy test - with it running spray carb cleaner around all the joints and over the carb. If it revs up, you've got a leak.

 

Also check the governor isn't stuck.

 

I did find mine was very sensitive to stale petrol. I remember having had huge trouble starting it one day. It was very low on fuel and because I didn't want to kill it again I naughtily I topped up with a couple of jerrycans with the engine still running. About 30secs later the engine note picked up. It started fine after that.

[R Cubed]

dont think I have any vac leaks as I have carried out a vacuum test and with engine hot at idle I am getting 20 inches Hg and a hard rev drops to nearly 0 then rises to 20 again decellaration after hard rev give 24 inches of Hg. No fluctuation of needle at idle either, the govenor use to rattle a bit at tickover and now you mention it I have not heard it for a while, will look at that tomorrow, thanks for jogging the memory about it..

 

On the subject of fuel do you prefer a particular garage, BP, Shell etc.

 

R Cubed

[Jack]

I am stupid, well so many friends say, but am I getting parranoid, my GMC 352 seems to be sluggish to get to 45MPH can any one give me some idear's of how long it takes to get to 45MPH from a stand still but under normal acceleration and flat roads, or say from 20 to 45, it just seems the throttle pedal is on the floor all the time to get it up there eventually, I dont know if the engine is down on power or is it that I have not driven it for such a long time.. Just a warning dont use the truck speedo mine is 5 to 10 MPH slower than Tom Tom says :dunno:

 

Feedback eagerly awaited

 

R Cubed

 

 

 

Hi.

 

Not to sure of your problem but I can share my experience with you if that helps. I have never has my GMC above 35 mph and the first time I got to that speed was when we were coming back from Salisbury Plain a couple of weeks ago................I have had my truck for nearly two years!

 

I not sure if I could get up to 45mph but I don't think I would want to as at this moment in time when I hit 35mph my truck lets me know that we are doing this speed, if you know what I mean. Until my truck has a total ground up rebuild I don't want to 'push' it to see where its limits are - there are a lot of props and UJs flying around down there.

 

I find the acceleration is wonderful with a lot of bottom end torque and pulls up hills a treat - the engine sounds beautiful when working hard. I am not to sure how many folks here push their Jimmy's up to 45mph as I would think that you would need optimum conditions i.e. slightly down hill with a quite a few miles of straight clear road in front of you as you will know - the GMC isn't exactly over powered (4.5l engine) and the slightest incline with drain the power even before you change down.

 

Hope that helps..........

 

Best wishes.

 

Jack.

 

 

[Adrian Barrell]

I used to run my GMC at 40 mph simply because I got 9mpg and it ran at 170 deg. If I ran it at 45, which it did quite happily, the temperature went up to 180 and the fuel consumption also suffered! Driving any slower than 40 made it hard(er) to get up hills etc. Using the momentum of the vehicle is half the battle.

 

One thing to check is ignition timing. A lot of people set the distributer according to the manual forgetting that setting is for something like 72 octane. Modern fuel allows you to advance the timing quite a bit and it makes quite a difference to power output and fuel consumption.

[Jack]

 

 

One thing to check is ignition timing. A lot of people set the distributer according to the manual forgetting that setting is for something like 72 octane. Modern fuel allows you to advance the timing quite a bit and it makes quite a difference to power output and fuel consumption.

 

 

Adrian - that sounds interesting, how is it down and how would you know that you have done it - once you have done it :dunno:

 

Cheers

 

Jack.

[TooTallMike]

I used to run my GMC at 40 mph simply because I got 9mpg and it ran at 170 deg. If I ran it at 45, which it did quite happily, the temperature went up to 180 and the fuel consumption also suffered! Driving any slower than 40 made it hard(er) to get up hills etc. Using the momentum of the vehicle is half the battle.

 

 

My 352 also cruised very comfortably at a verified 40mph on the motorway and achieved better fuel economy like that than my friend's one on the same route doing 30.

[Adrian Barrell]

Adrian - that sounds interesting, how is it down and how would you know that you have done it - once you have done it :dunno:

 

Cheers

 

Jack.

 

Well Jack, probably the first thing to do is establish what your engine is set to. Get hold of a neon timing light, find the original timing marks, which are on the damper ring in front of the lower pulley if I remember correctly and mark them with white paint or tipp-ex. You should have DC or TDC to indicate top dead centre for No 1 cylinder on the firing stroke i.e with both valves shut. There should be some other lines marked to indicate degrees before TDC to time the spark to.

 

You clip the pick up over the no 1 cylinder spark plug lead with the arrow on the pick up pointing towards to spark plug. Then connect the two small leads to a battery. Probably needs to be a 12V supply and doesn't need to be the vehicle battery. Making sure all the leads are clear of any rotating bits, start the engine and shine the neon light at the timing marks. Mind the light and your fingers in the fan! The light will flash and illuminate the marks making them appear stationary.

 

This shows you your current ignition timing, assuming the pointer is not bent out of position..... With the engine stopped, loosen the distributor clamp at its base just enough to allow you to turn it. With the engine running, turn the distributor in the opposite direction to the rotor direction, usually anti-clockwise, to advance the ignition. If it was a bit retarded, you should have a small increase in idle speed. You should be able to run at 4 to 6 degrees more advance than the book figure. But..... if you advance too far, you will get pre-ignition. This is bad! Also known as pinking, you can hear it under load as a ticking, ringing sound. This will damage your engine and it may also be hard to start if too far advanced. It's best to advance a little and try it on the road.

 

Sorry if you know all this already, but it worked well for me.

[R Cubed]

Yes thanks for all the advice, I am aware how to time it but have a problem at the moment that I cant move the distributor :dunno: even with all the bolts out, I can live with this at the moment but will investigate later, might need a new distributor if I have to smash it to get it to move !!!!!!!!!!!!!!

I thought the timing marks were on the flywheel represented by a ball in a hole to mark TDC or is it 5 deg BTDC ? I will also look in to this.

 

Seems to be getting better can easily get 35 MPH I recon it could be bad fuel, will get some BP stuff soon and try it from there..

 

R Cubed

[G506]

Morning everyone, my first post on HMVF!

Hi R3,

I cant remember, but is the CCKW fitted with throttle cable or rods/linkages? The reason I ask is very occasionally Ive seen engines (fitted with throttle control rods) unable to achieve higher revs due to a combination of worn engine mountings and incorrect rod adjustment, as the engine's inertia causes it to tilt to one side it travels too far, effectively preventing the full travel of the throttle mechanism. I doubt this is your problem, but may just be worth a quick check.

Cheers

G506

 

[Jack]

Morning everyone, my first post on HMVF!

Hi R3,

I cant remember, but is the CCKW fitted with throttle cable or rods/linkages? The reason I ask is very occasionally Ive seen engines (fitted with throttle control rods) unable to achieve higher revs due to a combination of worn engine mountings and incorrect rod adjustment, as the engine's inertia causes it to tilt to one side it travels too far, effectively preventing the full travel of the throttle mechanism. I doubt this is your problem, but may just be worth a quick check.

Cheers

G506

 

 

 

Welcome G506.

 

That is a dam good point as there are linkages as I will look closer at mine as I know for sure that there is some movement in mine.....

 

Cheers

 

jack.

[R Cubed]

Welcome G506.

 

That is a dam good point as there are linkages as I will look closer at mine as I know for sure that there is some movement in mine.....

 

Cheers

 

jack.

 

 

Hi and welcome G506, nice to see you got on here in the end ;-)

 

With my little trips out in the past couple of weeks it seems I was imagining it, on flat dual carridgeway my truck will whizz along at 40-45 MPH at only about half throttle and all is steady and quiet on steering and transmission, so a well happy bunny :-D :-D :-D :-D :-D

 

R Cubed

[rosie]

I kept telling him he was imagining it!!!!!!!!!! Glad it goes well now hunny ;-)

[Jack]

I used to run my GMC at 40 mph simply because I got 9mpg and it ran at 170 deg. If I ran it at 45, which it did quite happily, the temperature went up to 180 and the fuel consumption also suffered! Driving any slower than 40 made it hard(er) to get up hills etc. Using the momentum of the vehicle is half the battle.

 

 

 

 

Just an update on this.

 

I took the GMC up to Hampshire this morning and first time that i have taken it on a M way.

 

I was very interested he putting in to practise Adrians advice on speed and momentum and also taking advice from you guys with regards to getting the gear change correct and smooth. I also wanted to give a good hard run as I thought that if some was seriously wrong then it will be on this run that something would go bang.....

 

I put all advice in practice and I must say that the GMC went like a rocket - I went up to 45MPH and it cruised there mile after mile and at one point it was knocking on the door of 50MPH - incredible - never thought that that could be done. Over the moon it was more than happy, no wheel wobble no nothing, drove a perfect line. And can now understand the importance of momentum - it just pushed right on up the hills.

 

Could I driven staright on past Rich and carried on to Suffolk.

 

This is what this site is all about - thanks for the advice.

 

Jack.

Speed Limit Breaker

 

 

[R Cubed]

Hmmmmm since convoying at Bolero I seemed to have trouble to keep up with other GMC's on hills ect they seam to just steadily pull away, in the next few months I am going to sort this out I think its a timing problem, having driven Jacks truck it seems to rev very freely and pull well from low down even on 4 or 5 cylinders, I am mistifyed by this, more investigation is required stay tuned...

 

R Cubed

[Jack]

Hmmmmm since convoying at Bolero I seemed to have trouble to keep up with other GMC's on hills ect they seam to just steadily pull away, in the next few months I am going to sort this out I think its a timing problem, having driven Jacks truck it seems to rev very freely and pull well from low down even on 4 or 5 cylinders, I am mistifyed by this, more investigation is required stay tuned...

 

R Cubed

 

 

This is interesting Rich, epecially with regards to my GMC - I think it has been running correctly for a while but it does pull nicely. Lawson (my brother in law) has just brought a GMC as you may know and we picked it up at the weekend - you need to drive this one at Upottery as this is a beautiful engine and pulls like hell.

[R Cubed]

Ok I would love to see how another GMC goes and also what it sounds like in the cab transmission noise !!!

 

Will see you there.

 

Cubed

[N.O.S.]

First posting by New Old Stock - Hello to all of you!

 

For what it's worth:

 

I spent a long while driving my 353 closed cab around (at 30-35), becoming more and more paranoid about even the slightest whine or rattle. When I eventually got to be a passenger in the back I could not believe what a beautifully smooth sounding engine and transmission it had! :-D A highly recommended experience if your'e getting worried.

 

Only real noise was lumping and bumping when turning sharp at low speed or running on bumpy surface - no play at all in axle tie bars, but I put a new rubber bushed set on anyway and what a transformation - trouble is new tie bars are not cheap any more....

 

High speed running - came oh so close to visiting the ditch one day after fitting new tyre/rim assemblies (off ben hurr trailers) - front wheel balancing cured it but frightening how the wobble builds up to be almost uncontrollable.

 

I've heard mention that balancing propshafts will transform the ride of a jimmy - anyone had luck with this?

 

Sorry if these points raised before, it is taking a long while to work through all your postings - so much useful information in here, some great advice, tales and dark humour.....

 

New Old Stock (think the water might have got in though.....)