16 replies to this topic

[hhhh]

I have a Canadian '79 998 in my daily runner as well as a spare 998 that seems to be identical with about the same mileage. I'm about to upgrade the head to a 0.100" milled and ported exhaust 12G295, exhaust to original pressed steel Cooper with 1-3/4" pipe and glass pack, carb to HIF4, and rockers to 1.3:1 forged, and decided to check how much a worn timing chain matters to timing. It turns out my spare 998 has full intake lift at about 117 degrees ATDC which is 11 degrees retarded from stock timing of 106, if it's the standard the cam which I believe it is. It has the same lift of 0.235" for exhaust and intake which doesn't match what it's supposed to be (it should be 0.250" intake lift supposedly.) Is there another cam that could have these lift specs that came from the factory?

 

The horribly retarded timing probably explains the somewhat lame low end torque, yet it will still go close to 140 kmh. It's so retarded that skipping a cam sprocket tooth will actually mean it's still 2 degrees retarded! I think I'll convert to duplex. Since my lift is so low, does anybody think that 1.5:1 rockers might actually be a good idea on this small bore? I know they're generally considered a good way to lose low end torque on these motors, but I have a feeling my other upgrades will not be realized without a bit more lift than 290 at the valve.

 

Carb needle suggestions are welcome. I have an ABC, an AAE and an AAG to play with for a start. My main goal is economy but increased power is a bonus. Would the 1.5 rockers in this low cam lift context really hurt mileage? Thanks for any comments.

 

[Dusky]

Factory timing is 110*.
Time the cam in 2 degrees advanced to take up chain stretch etc. So skipping a sprocket and setting the timing at 108 is ideal for you.
On terms of a good cam: sw5, kent 266, mg metro

Forget 1.5 rockers, on a 998 they cost power unless you race it.
Cheers

Edited by Dusky, 09 September 2015 - 05:46 AM.

[jaydee]

In my experience a 3° retarded timing gives a better low end torque on small bore engines.

You can change the cam woodroof key to fine adjust timing anyway.

[hhhh]

Thanks for the replies. I have Vizard's old blue book and the cam in the second column of the factory cam chart states 106 as the stock timing figure. I should have clarified, for now I plan on using the stock cam, so I would expect the 1.5 rockers to actually help with its pathetic lift of 0.235" both intake and exhaust to bring valve lift to around 0.260" which certainly is not extreme. I'm not sure why my exhaust and intake lift are identical. Maybe the Canadian versions got a special "pollution control" cam or something with low intake lift.

 

Jaydee, I'm surprised that a retarded cam could give better low end; that defies standard engine wisdom, but I'll take experience over that any day. It of course depends on whether it's wrong to begin with. Cheers.

Edited by hhhh, 05 April 2017 - 01:23 PM.

[jaydee]

998 engines and 1275 engines do not behave the same, you live and learn

All BMC cams have to be timed at 110..however as vizard said on his book, they work better timed at 106°. Indeed the kent megadyne range was developed experimenting from standard cams with uprated rockers and advanced timing.

Anyhow, on a small bore engine a 1.5 ratio will never work properly, valves are opening and closing too fast for the lift.

Edited by jaydee, 09 September 2015 - 02:14 PM.

[hhhh]

Okay, so the problem isn't about too much full lift, it's on the lead up and down where the valve acceleration is too fast to maintain gas velocity on the small bores. That makes sense. Thanks.

[Spider]

As Dusty and Jaydee has mentioned all the early A Series Engines had their cams timed at 110⁰ however in later years, interestingly, many of the engines, while running similar profiles had their cams advanced to 106⁰ and 107.5⁰.

 

Generally - and I stress that - generally - advancing  cam timing improves the low RPM power (this is why the factory did it as they were using taller Final Drives too at that time) and retarding it, gives more power higher up the rev range, this is usually why worn engine seem to scream at high revs (worn cam timing drive).

 

How are you measuring your cam timing angles and the lift?

 

If you are in fact finding that the Cam Lift is short of the original specs by some ~0.015", then sorry to say, but fitting high lift rockers is not the answer here, the cam is worn and a replacement (or possibly a regrind on it) is the only way around that. One thing I have found over the years is that when new, the factory cams are surprisingly very accurate, I don't think I've found one more than 1⁰ out in terms of angle and all I've measured have had as advertised or maybe 0.002" over advertised lift. Used with the right oils and having the oil at the right temp, they also last very well too.

 

I'd actually be inclined to suggest fitting it back up with stock rocker gear and then trying the 1.5 rockers, easy engine in swap job.

Edited by Moke Spider, 09 September 2015 - 07:51 PM.

[hhhh]

I'm measuring the cam timing in the usual fashion with a dial gauge off of a pushrod with the head off and checking the degrees in the forward direction (unloaded by valve springs of course) at 3 thou before peak and 3 thou after, summing and dividing by two to get the lobe center. I really doubt that every intake lobe is worn to exactly the same lift as the exhaust lobes, it just doesn't seem likely.

[Silver25]

For comparing needle differences, mintylamb is the best site by far.

http://www.mintylamb.co.uk/suneedle/

I just start with the needle I have, then compare ones from the lists until I find one that will give me the right fuel distribution I'm after.

Hope this helps.

[hhhh]

I just checked ACDodd's cam post ( http://www.theminifo...identification/ ) and it seems my cam spec matches the "Low CR 998 A+ cam." At least the lifts match better than anything in Vizard's blue book. I suppose it's possible they fitted an A+ cam to the '79 Canadian 998s.

[hhhh]

In my original post, I was in error regarding moving a tooth on the cam sprocket, It would be an 18 degree change, not 9 degrees. The easiest way to visualize this is that a complete cycle of 720 crank degrees is 40 teeth (two turns of the crank or one turn of the camshaft.) Therefore each tooth is 720/40 = 18. Regardless, I just got a great deal on a barely used vernier duplex sprocket set, so that problem is gone. I also went through my junk and found an 1100 cam carelessly tossed into a box but in great shape, so I'll be running that instead for about 0.020" better lift and more duration on the exhaust at least.

[hhhh]

Tore down my motor yesterday and found out I had a cam chain tensioner as fitted to A+ engines, but my engine is certainly not an A+. The tensioner, although there, wasn't tensioning anything because it was backed off too far to even touch the chain. What's the maintenance interval for retensioning?

 

I confirmed that this cam is identical to the cam in the other engine I previously measured the lift on with 0.235" for both intake and exhaust, although that engine didn't have the tensioner. So what's going on here? It looks like BLMC made some hybrid engines in the changeover period from A to A+, at least for the Canadian market.

 

Has anyone seen this type of motor in the UK? The engine number starts with 99H8 which doesn't appear in any ID chart that I've seen.

Edited by hhhh, 12 September 2015 - 07:06 PM.

[Spider]

The tensioners started appearing around 1976 on 1275 engines and about late 78 on 998's.   I did briefly consider that Cam in my first post here but I felt your engine was too early to have that one fitted (paperwork I have suggests it wasn't fitted or even manufactured until about 84-ish, but there you go).

 

I'm not aware of any tensioner adjustment interval published by the factory, however I always fit them as tight as they will go. Sounds like your chain and sprockets are in need of replacing in any case.

 

The Engines that you guys got around that period were the same as we got here to comply with the local emission rules. I've actually had that confirmed to me by both the Foreman of the Engine Shop and the guy that did all the Technical Research at our end that rather than have another engine spec assembled in the UK, while not 100% ideal, they settled on the Canadian Spec engine (which is also the same as that for Sweden at the time).

 

The Engine No. Prefix would have been something like 99H860AJH, 99H883xx or 99H889AH

Edited by Moke Spider, 12 September 2015 - 09:16 PM.

[hhhh]

Thanks Moke, very informative. I've changed it out to a vernier duplex setup, dialed the 1100 cam in to 106 degrees to optimise low-end torque and I'm getting about 0.325" lift at the valves now, a huge increase from the 0.275" I had before. Hoping to finish and fire it up tonight.

[gazza82]

hhhh: is your vernier gear tight to move even when the allen screws are very loose?

 

Split mine to give it a good clean up (and did make a note the orientation) but the inner and outer parts are really tight and hard to move.