The Triumph Dolomite Club - Discussion Forum

Hi all.

Just had Sprint head rebuilt with a cam and rockers from another car so - line boring and top face and bottom face levelled plus shimming up. Not my idea, theirs.

Put the head back on and it runs but bad - it hunts like the timing is out. I check everything over, air leaks etc etc. all ok. I check the valve clearances and i have 8/9 ex and 10/11 in which is very tight compared to the 18 the manual says. Crank and cam all line up damn near perfectly.. no amount of dizzy twiddling improves matters.. changed the cap and plugs too..

So, would the tight clearances account for the really rough running or should i look at something else first? All the head work will have altered the cam position too so that might also impact.

thanks for any help. looking for a second opinion before i have to pull it all off again

Too tight clearances will alter the valve timing. The valves will start opening early and close later.

Search and ye shall find......

viewtopic.php?f=4&t=32015&hilit=valve

GTS290N wrote: ↑Fri Sep 21, 2018 6:21 pm Search and ye shall find......

viewtopic.php?f=4&t=32015&hilit=valve

That thread suggests the OPs problem isn't the valve clearances.

Are you sure that everything else is OK? I had an engine that ran like a dog and it turned out the ebay HT leads I was using were garbage. Once I switched them for better ones, the problem went away.

thanks - i'd seen that post.
I'm running a std cam.
Car ran like a dream beforehand - the only thing changed was the head..

the old post suggest tight clearances are possible, but will affect valve timing and it isnt clear how you can change the valve timing other than readjusting the chain?

Modern thinking is the tighter clearances are ok so I dont think its that. So its either bad ignition or fueling imo. Does the engine smooth out a bit when bringing the revs up to 1500 or more?. Check a good earth from distributor to block as a basic check. How does the timing light flash look? Intermittent could show a bad lead or cap/rotor. Try all plug leads with the light connected to see if theres a difference in "flash".
Are the carbs balanced? Are the fuel bowls equally fully when you lift the bowl tops off?
Tony

tony g wrote: ↑Sat Sep 22, 2018 2:32 pm Modern thinking is the tighter clearances are ok so I dont think its that. So its either bad ignition or fueling imo. Does the engine smooth out a bit when bringing the revs up to 1500 or more?. Check a good earth from distributor to block as a basic check. How does the timing light flash look? Intermittent could show a bad lead or cap/rotor. Try all plug leads with the light connected to see if theres a difference in "flash".
Are the carbs balanced? Are the fuel bowls equally fully when you lift the bowl tops off?
Tony

Check, check, check and check! Been through it all with a second and trusted pairs of eyes and now we're scratching our heads. I thought maybe i'd introduced slack in the chain when i took it all off, but everything lines up fine. Interesting, i put new plugs in it and 1 and 4 came up looking a lot cleaner that 2 and 3.. Is there any other way checking the valve timing is right?

A quick check is to put the crank at tdc as shown on the pulley and timing mark and with plugs removed check the piston is at the top. This will verify that the pulley is showing correct tdc. Sometimes on balancer type pulleys they can spin on the rubber which may show a false tdc reading and this can throw the cam timing off of course.

Tony

tony g wrote: ↑Sat Sep 22, 2018 9:15 pm A quick check is to put the crank at tdc as shown on the pulley and timing mark and with plugs removed check the piston is at the top. This will verify that the pulley is showing correct tdc. Sometimes on balancer type pulleys they can spin on the rubber which may show a false tdc reading and this can throw the cam timing off of course.

Tony

Thanks Tony - its spot on by the look of it. The only thing i cant accurately account for is the valve open / close timing which is why im concluding that the clearances and re-position of cam are sufficient to throw the timing out and create the 'hunting' it does

There are a number of ways to check TDC and lucky for us a 4 valve means a plug in the middle and I use a long 1/4 drive socket extension and in my case a DTI (dial test indicator or clock). Once TDC on the no 1 firing stroke has been established the cam can be checked and the groove should be in line with the cam bearing slot.

From what you have said, I would bet a jam sandwich on it being a tooth out on the chain drive, as I always use tight clearances on the engines I build, with no adverse effects, apart from being quieter of course.

trackerjack wrote: ↑Sun Sep 23, 2018 9:55 am There are a number of ways to check TDC and lucky for us a 4 valve means a plug in the middle and I use a long 1/4 drive socket extension and in my case a DTI (dial test indicator or clock). Once TDC on the no 1 firing stroke has been established the cam can be checked and the groove should be in line with the cam bearing slot.

From what you have said, I would bet a jam sandwich on it being a tooth out on the chain drive, as I always use tight clearances on the engines I build, with no adverse effects, apart from being quieter of course.

Hi - Yes, i've done that check and defo have TDC with both pulley and cam lined up.. so, if i am a tooth out on the drive, wouldn't that put the cam in a different position? (thinks=valves smashing up pistons) How do you know its clockwise or anti pls and how would you move it round one tooth exactly?

appreciate all the help here folks - its raining out so im very tempted to get the head off today!

If youre happy with TDC and when turning it to tdc in the normal direction of rotation (important), and you can see the line on the cam is lined up with the line on the cam carrier then cam timing is correct. A tooth either way wouldnt cause valve clashing and would run a bit "off". If it was a tooth out you would have to release the sprocket from the chain and move it round the chain by a tooth the re-attach to the cam. Moving the cam back and forth a bit to line up the bolts is ok to do. When fully attached move engine backwards a few degrees and creep up on tdc again to check the lines are matched. Turning backwards puts the chain slack on the opposite side and gives a false impression of being lined up correctly. HTH

Tony

philwesty21 wrote: ↑Sat Sep 22, 2018 9:10 am thanks - i'd seen that post.
I'm running a std cam.
Car ran like a dream beforehand - the only thing changed was the head..

I hate to say this, but if everything is the same except the head and it is all setup right.

Is it possible there is a flaw in the head?

Have you done a compression test?

Thanks for the help.

I think the tooth out diagnosis sounds about right so I'm going to pop the timing cover off and do a belt and braces re-alignment.. Have protractor, will use it. Much easier to do that than take the head off again.. inching the chain round a tooth will be a lot less stressful too

i did check the compressions - fairly even 140 when cold.

Will report back when i get onto it later in the week!