Blowback.....could this be MJ related?.

[Hiperformance71]

Hello Paul,

If Now you have definitively eliminated the "ignition factor", right, you have now a mechanical problem. A friend of mine, a mechanic who propare many racing cars with carbs and distributor, two months ago have had a similar problem but at higher rpms, he tried some possibilities for resolve this problem, he tried increasing the length of the runners (about 1 cm extra), re calibrated the advance/retard of the Camshaft (single) and the inclination of the carbs, After some tests he has founded the right combination to solve the problem and finded more power too.

The cams do you have, are well indicated for your engine state of tune?
Some cams are more indicated with certanly engine mods, with others mods they don't made the optimal potential. If you can advance/retard the cams by a little, you can investigate the engine reactions. The cams is best to install using the wheel degree placed in the Crankshaft and using the gauges for find the TDC of the cylinder 1 and the correct aperture of the valves, take a look at this articles (my english isn't soo good for explain this important procedure):

http://www.hotrodder.com/kwkride/degree.html
http://www.ridgecrest.ca.us/~biesiade/camdegree.htm
http://buickperformance.com/camdegree.htm
http://www.thedirtforum.com/degree.htm

cheers,

[Jenko]

Hyperformance...you are spot on....sort of...

This weekend, I thought I'd start stripping the engine....so I first whipped the cam out....Guess what....It was totally worn!!!!. some of the lobes were 5mm shorter than they should have been, which, when amplified buy the rocker ratio, meant some valves were at least 8-9mm short on lift.

Everything else in the head was in prestine condition...the car has done 900 miles since rebuild (at which point a brand new cam kit was installed), and has had 3 oil changes...

Lifters were perfect, so it point's to the cam not being hardened correctly...I have been speaking to the supplier, who has asked for it back to look at...

Problem was...due to the engine design it was easy to look down the lifter barrel and see the cam lobes...they appeared to be in perfect condition, but obviously I couldn't see the profile......I actually feel a huge sense of relief as at least I know what it is now....Many many thanks for everyones concern and help, it has been very much appreciated...I'll keep you updated on the customer service of the cam suplier

Paul.

[david jenkins]

Paul,

As a matter of interest, did you run-in the cam when you first started the engine?

Generally, a new cam should be run at 2500 rpm or higher for the first 20 minutes - see Burtons for details (see first and last paragraphs for relevant bits). A great way to impress the neighbours, especially if you haven't fitted the silencer/muffler yet!)

The main aim is to get a strong flow of oil at this critical time. Also, at this rpm the followers run correctly over the cam lobes, rather than having to drag them up and over the slopes.

There is a high risk of premature wear if you don't do this...

regards,
David[/url]

[Jenko]

Hi David,

Totally agree, and I was 100% meticulous when running in the engine and cam....2000rpm for 20mins...it hurt to do it as it just doesn't seem natural...but I understand the theory, so thats what I did...also made sure the cam was very well lubed with the manufactures cam lube....all done by the book....

I spent a fortune building the engine...so certainly did not take any short cuts.........I honestly believe is a case hardening issue...but cam has now gone bak to manufacturer, so will see what they say.

Cheers.

Paul.

[aarc240]

Nice to see that instinct & experience telling me that mixture bouncing of the back of the valves was the culprit but sorry that the cam was so bad.
Just curious - are you using cam followers from the same manufacturer as the cam?
Replace the followers as they will have copped a hiding from a cam that soft. Funny angles on the contact face and other nasties are possible and used followers are a sure way to lose a new cam and even an engine.
If possible, get followers known to be compatible by the cam people.

[MartinM]

It was totally worn!!!!. some of the lobes were 5mm shorter than they should have been...

And where has all that metal gone? I hope it's:
- in the oil filter
- in the discarded oil from the three changes
- ...and not in the current oil/big ends/cam bearings etc

[brentp]

That is incredible. It might be worth performing an oil analysis and checking other areas for obvious signs of wear, like the cylinder walls.

How many oil changes did you make since the engine was put back together?

[Jenko]

aarc - to answer your question...yep, everyone who siad it was valve timing was spot on....I'm hitting myself, as the last thing I though it would be was a brand new cam at fault....The lifters were bought as part of the cam kit, so came from the same manufacturer.........

Martin, as for oil, I've thought about this.....Thanks god I changed the oil so often (actually, it was 4 times)!....I think thats given me a fighting chance....And every time, I've changed the filter.

Needless to say, the engine will now get a full flush out before running it again...as the lobes had worn so smoothly I'm hoping there is nothing large in the engine (I doubt it), I also, did some working out about how many CC's of metal has worn away, and in the grand scheme of things, when diluted into 4.5ltrs of oil...it's not as bad as it sounds....I hope

I'll keep you chaps updated as to what the cam people say

Cheers.

Paul.

[Jenko]

Hi Chaps....

Just wanted to give you all another update.....

I bought a rather nice cam and solid lifter set up from flea bay last week, so while the other cam has gone off for inspection I fitted said new cam (after flushing engine through twice!)....

Anyway, upshot was, car now revs easily up to 7krpm, and makes very good power up to 6.5krpm.....just felt really good to get the thing finally working....

Once again, thanks to all your inputs, I know it's not MJ related, but I hope it made for interesting reading, and got the grey cells working

Cheers.

Paul.

[Dave999]

what kind of oil did you run on the run in proceedure

modern oils lack some of the ingredients necessary to run in a new cam, especially when you up the valve spring tension
i think its litium and molibdenum (sorry spelling)

its all Health and saftey stuff that affects general sales but not manufacturing (so car manufacturers get the good stuff for run-in we don't)
This has all come about in the last 5 years

any modern oil will allow a cam lobe to be wiped in minutes once you stray from standard spec on cam lift and spring tension.
They are so good the parts never touch hence the work hardening necessary for extended cam and lifter life doesn't happen

you need a running in oil so all moving parts get a bloody good hammering

or literally paste the whole lot in as much assembly lube you can.... pump it in with syringe

never run in on synth or semi synth

also if you can't mechanically prime your oil pump
block off all breathers and have mate apply a compressed air line to the dipstick tube on start up that will certainly blows the oil out of the sump up the pickup into the pump ASAP

this is an issue plagueing the V8 guys at the mo. build yourself a nice Mopar 440 and had no valve lift after 20 minutes of run in... its rife.

lots of guys just use a bog standard multigrade mineral oil from chevron or texaco, real happy shopper dollar shop, old mineral oil, with a running in additive

costco do an 8 gallon drum of chevron for £15

obviously jetison the oil and filter after the first 20 minutes refill with the same and get rid after 1000 miles

And no pansying about either

up to 3/4 before redline back down again in first second third fourth take it up to 70 and slow it right down on the box drive it like you hate it for at least 50 miles.

we have to change with the times, no babying it round anymore

cos the issue affects ring bedding in as well if you don't get a nice up and down face worn in in the first 1000 miles it affects power output and engine longeviity for the life of the engine

less oil contamination with well bedded in rings.


fill it up with expensive juice once its loosend up a bit


dave

[aarc240]

Well, I scratched the old head for a bit on this one from Dave999 and finally decided some of these old tales really need to be put out to pasture.

1. normal oils never did contain moly or for that matter any significant quantity of lithium. They were in greases, each for a specific use.
2. current manufacturers don't use some exotic oil for running in, they just machine and finish everything properly. Also of interest is that it is common practice to do the first fire up using natural gas for the fuel.
3. cam lobes and lifters do NOT work harden. The steel alloys used in manufacturing them require heat treatment at several hundred degrees Celsius in a cyanide salts bath. The ONLY steel alloys that exhibit a surface work hardening are some of the stainless and austentic ranges (and no-one is mad enough to use that stuff for cams)
4. use the assembly lube as suggested by the cam / lifter maker. It contains moly (among other exotics) and lots of it in the engine WILL get into the oil and then onto the bores which WILL prevent properly finished bores and rings seating. After all, moly is a bloody good lubricant!
5. if you really must use synthetic oil (or semi) from initial startup, go to an experienced specialist machine shop for all your machining. You CAN run-in on synthetic but the bore preparation and finishing is VERY different.
6. always do the initial bed in at 2000 RPM for 20 minutes, change filters and oils and then drive it just as you intend using it in future. Never let it lug under heavy load at low revs but also don't allow it to spin-up with no load.
7. getting a poor 1970's standard of build followed by a hard run-in will never equal the results of a modern plateau honed bore followed by a sensible bed in for either power or oil consumption.

Of course, if you absolutely insist on using dinosaur technology like chrome faced rings etc then you will have to treat it like a dinosaur.

Another old tale that is worth discarding is the one about running in LPG (propane) fuelled engines. A lot of 'experts' would have you beleive that the rings won't seat properly unless the run-in is done using petrol (gasoline) for at least 1000 miles. It's a load of BS! A properly machined and finished engine WILL bed in very quickly and very well on LPG. In fact the LPG only 2.8l OHC Nissan in our tarmac car achieved almost unmeasurable compression leakdown in less than 600km!

btw, LPG / propane engines are very much benefitted by the MJLJ / EDIS combination.

[Jenko]

Hi Gents,

Just FYI, I ran in the cam exactly to the manufacturers spec......I used a Mineral oil (also coated cam / lifters with cam lube), and upon first start up, went to 2000-2500rpm for 20mins.......I probably did another 50 or so miles without letting the engine dip too low in the revs, and changed the oil and filter for some 10W40 Semi Synth......After another 200miles I changed oil again

So far so good with the new cam.....No probs yet

Interesting posts on oil though

Paul.