1976A Dyno Run

[oldjapanesebikes]

I blew out one of the crankshaft seals on my blue 1976A and so a rebuild is planned for this winter. After several discussions with Richard, I've decided to use a lift plate and do some cleanup of the ports, skim the head and see what sort of a power boost I can get. Before doing so, I thought it would be useful to get a 'before' shot of what the engine was putting out so I'd have something to compare a fresh engine to !

I've had my blue 1976A almost as long as my red 1976A - late 1980's and I use it as my parts bike or mule for testing oil pumps, carbs etc, or as a place to hang good spare parts on till I need them. As such it is a bit of a 'bitsa' in some respects, but the engine has always been solid. At the moment it is using stock BS40 carbs, stock airbox. points, stock exhaust, the bottom end and transmission is original, standard bores and pistons, the barrels were changed in the 1990's and some very mild clean up done on the ports at the time. It is also leaking coolant from the water pump weep hole and smoke and oil billows out the right side pipe. I checked the timing and carburettor balance, changed the plugs and gave it some fresh gas before the run. Here is how it looked on the dyno



The sheet is to try and catch the gearbox oil being blown out the pipes.

I wasn't expecting too much in the way of results and so was quite pleasantly surprised ! The best run of the five runs showed 55 HP at about 6000 rpm and maximum torque at about 5400 rpm of about 47 ft/lbs. Interestingly, the tachometer read about 240 rpm higher than the dyno recorded at 7000 rpm so it was a good correlation. Ambient air was about 16C. The engine actually improved slightly with each run.



After the rebuild (and more discussions no doubt with Richard) I'll re-run the dyno test and see what if any improvement I get ! Stay tuned.

[Craig380]

55bhp at the wheel is VERY respectable for a tired motor on the verge of a rebuild!

And that smokescreen is EPIC

Suzi strokers do still perform even when they're well worn. When I first tore down my own 380, the cylinders had wear lips that you could feel, and the piston ring gaps were ALL over the 1mm service limit but it would still put 95 on the speedo anytime I wanted.

After the rebuild with good jugs, matched pistons, new rings etc it was noticeably sharper in pick-up and midrange, and mechanically a lot quieter ... and would still put 95 on the clock anytime. So peak power was pretty much the same as before, even though it was quicker getting there after the rebuild ...

[T350guy]

55 rear wheel Hp is not bad when Suzuki Claimed 67 and the Kawi Claimed 72..both I bet were just advertisement numbers

[tz375]

55 is a pretty good number for a stock bike.

Not so sure about that smoke screen though. Either the bottom end is full of oil or the pipes need to be cleaned out. Maybe take it out for a through thrashing before the snow flies.

[ja-moo]

The early H2's made right at 64 at the rear wheel (stock)

[Suzukidave]

The early H2's made right at 64 at the rear wheel (stock)

Difference is the H2's were made to be hot rods by design .. not so with the GT's .

[oldjapanesebikes]

Not so sure about that smoke screen though.

At first I wasn't either - it got progressively worse last year whenever I took it out, always the same pipe, injector oil level holds steady in the tank and now the gearbox level drops steadily and the smoke never lessens.

At least it isn't likely to rust.

[oldjapanesebikes]

Rebuilt cranks are back - I just need to get the Cobra off the work bench so I can start on one of the GT750's .................

[JoostGT]

Did you have the cranks shipped in half a chopped up crankcase?! I can see why it's a good idea, but I've never heard of it before!

Interesting to see what you're going to achieve since I'm doing the exact same thing at the moment:)

[oldjapanesebikes]

Did you have the cranks shipped in half a chopped up crankcase?! I can see why it's a good idea, but I've never heard of it before!

I actually keep the chopped up crankcases just for storage - the fellow that rebuilds my crankshafts is just south of Calgary, so close enough that I can just drop them off. Some years ago I was looking for something to put a spare crank onto for an extended period and that would support all the bearings evenly and ensure the alignment stayed true till I needed it, and then I realised that I had the perfect crankshaft 'keepers' in my scrap bin waiting to go to the recyclers ! Problem solved !

....I'm doing the exact same thing at the moment:)

Well - looking at your work with the injector system I'd say you are in the 'pro' league while I'm more 'midget' league but thanks ! My end-goal is quite modest really. I just want to see what actual, measurable improvement I can get with some minor mods, stock pipes and BS40 carbs. I'm also interested in what difference pipes actually make and so my plan (always good to have a plan even if it never happens ! ) is to do some dyno runs with different pipes after I get the new engine broken in next year. I have the stock pipes, a set of Gibson's, a Strader, access to what I'm told are a set of Tomcat's and (just for laughs) a spare set off of a J.

[JoostGT]

....I'm doing the exact same thing at the moment:)

Well - looking at your work with the injector system I'd say you are in the 'pro' league while I'm more 'midget' league but thanks !

I think that everyone who rebuilds their own engines and has the courage to do so deserves more than the moniker 'midget'! You have to remember that there is an incredible amounts of people who never even heard about crank seals, port timing and expansion chambers.

My end-goal is quite modest really. I just want to see what actual, measurable improvement I can get with some minor mods, stock pipes and BS40 carbs. I'm also interested in what difference pipes actually make and so my plan (always good to have a plan even if it never happens ! ) is to do some dyno runs with different pipes after I get the new engine broken in next year. I have the stock pipes, a set of Gibson's, a Strader, access to what I'm told are a set of Tomcat's and (just for laughs) a spare set off of a J.

This I'd like to see! This is what I meant by doing the same, I have the same goals (atleast for now!) and I'm always interested in reading about someones research. And since you're doing such a thorough job with your website I suspect your work with this will be just as good.

[oldjapanesebikes]

Just an update - so the engine was rebuilt with the lift plate, no porting but some light clean-up of the transfers, inlet and exhausts and then a few hundred miles of breaking in. This past week I took it back to the same place as before for a re-run, and the results were interesting. One can always argue whether the dynamometer readings are real - I seriously doubt that results are exactly reproducible from one dyno to the next normally, but I do think that they are useful as a comparative tool if done on the same dyno. So this is back on the same dyno as before, same dyno software and the same dyno operator - the differences are the correction factors for the weather conditions the days the tests were done, the condition of the engine and the different exhausts.

Here is what I have so far:

• - the lower line is the first test run before the engine rebuild - topped out at 55 HP and then dropped off to 53 HP at the rear wheel, corrected torque was 47 ft/lbs

• - the middle line is the rebuilt engine, with a cleaned but stock exhaust system - it topped out at 63 HP (rear wheel), corrected torque was 50 ft/lbs

• - the top line is the rebuilt engine with a set of Gibson pipes - it topped out at 70 HP (rear wheel), corrected torque was 57 ft/lbs

All runs are with a stock airbox and filter, stock jetting and BS40 carburettors. I did change the ignition to an Accent unit for runs represented by lines 2 and 3, and the bores are on first over for lines 2 and 3, so the engine displacement is now actually closer to 750cc rather than 738cc. The lower line (line 1) was on points. I still have four other exhausts I could test and I will post up the run results for any other exhausts when I have them. At the moment though, I am both pleased and also surprised at the results using the Gibson pipes with the lift plate engine - a gain of 15 to 17 HP is definitely noticeable when riding the bike.

And a reminder - the objective of this exercise was to get some base line data that can be tied to specific changes that I've made - I'm not actually after maximum performance, although at some point I may tear the engine down again and actually do some mild porting just to see how well the results would correlate with prediction.

[Craig380]

Excellent stuff - I bet she feels peppier with 70+ bhp at the wheel, and 57lb-ft is a good chuck of twist, too

[Vintageman]

Looks like you gain powered across RPM. Great Street Bile! The Gibson's added to a steeper slope on the top end. Wonderful!

What year cylinder (1976)?
How much did you lift?
How did you restore compression: How much did you shave off what... what PSI compression did it read?
Stock Piston skirts length? Stock Ports as you said?
KPH = what in RPM?

Do you have Jemco's 3 into 3s you can test?

Thanks

[oldjapanesebikes]

What year cylinder (1976)?
How much did you lift?
How did you restore compression: How much did you shave off what... what PSI compression did it read?
Stock Piston skirts length? Stock Ports as you said?
KPH = what in RPM?

Do you have Jemco's 3 into 3s you can test?

I have included additional background and photos on my blog which is located on my web site (http://www.oldjapanesebikes.com/) or directly at this link - it is spread over three articles.

Specific to your questions though:

• - late 1976 engine and block

• - the lift amount was taken off the top of the block - in this case about 3mm

• - head was skimmed - compression is 150 PSI across

• - stock 1st over pistons (no changes), and stock porting - the ports were cleaned up a bit, but not dimensionally changed in any way. I did clean up the passageways a bit, especially where the inlet spigots fit to the intake boots

• - the pulls were all done to 7000 RPM - not sure if that answers your question ? I have additional charts, but the one I posted just nicely showed the three cases which is why I shared it

If someone wants to lend me a set of Jemco's, I'll post the results - I will be doing runs with a set of J&R's and a set of Straders which I will be posting at some point. I also want to do a run with a set of stock pipes with cross-couplings (so J/K or L) so I have something to compare the Strader to (I suspect the torque curves will be similar). I also have a couple of sets of the reproduction Delkevic pipes - as they do not have the acoustic dampening that is built into the stock pipes, I'm wondering whether they flow the same as the originals and may do a run with them at some point as well. At the least, it would be a way of confirming how similar they are to the originals.