A question for the tuners

[Dave8338]

When in doubt, go with Keith on the numbers. He brings LOADS of knowledge to the table, when it comes to these bikes, modded...no disrespect to all others who know ten times as much as I, about the GS.

I'm sure you've seen this somewhere else, but if not, it seems to be the most straight forward, method oriented, carb tune procedure that I've read, as it pertains to bikes. I'm a carb freak (mostly 650CFM and above)and have read numerous books/articles on various carb tune procedures, and as far as I can tell, when starting from scratch (heavy mods, one size does not fit all), this method will get you from start to finish and not leave you guessing. My .02 cents. http://www.factorypro.com/tech_tunin...m_engines.html

Dave

[49er]

OK. I guess 49er and I differ a bit about the necessary throttle positions to make the 3 basic jetting circuit tests at.
My opinion is to test the main jets at full throttle. Test the jet needle at 1/3. Test the pilot circuit at minimal throttle openings. Mark your throttle grip and housing if any doubt.
The idea is to try to eliminate any overlap effect between circuits. On your VM carbs, the jet needle comes into effect much sooner than many realize, especially with the different DJ needles. A minimal throttle opening will show you what the pilot circuit is doing with the very least possible overlap effect from the jet needle. All you need is the mimimum. Testing the jet needle at 1/3 also eliminates any effect from the main, which will kick in at 3/4 throttle.
There's lots of charts on this stuff that show when this and that kicks in. They can be different enough to confuse. For these VM carbs, I have my own chart in my head.

Yes, we do vary a bit on the throttle testing positions. We agree on the main jet testing proceedure. I agree that the Main jet is the starting point as suggested in the link provided by Dave 8338.

There have been numerous posts on this subject, and they wary greatly depending on the engines state of tune, and the resources available to the tuner. I intent making this my last post on this subject.

When we say 1/8 or 1/4 throttle, the actual amount of throttle travel at the grip will vary from model to model and bike to bike. Many GS owners report that they have up to 1/2 of a full grip rotation, when at full throttle. On my bike, I have 1/4 grip rotation at full throttle. It becomes quite difficult to tell when you reach 1/8 of full throttle, or 1/4 for that matter, just by feel. IMO, it's not a safe practice to mark the positions on the grip so you can visualise while riding/testing.

The reason that my throttle testing positions are a little higher than Keith's is because a believe that the pilot circuit does still have a significient influence on the smooth transition between the pilot and needle circuits.
If you have unlimited resources, you will have a bucket of jet needles with varying diameters and tapers that will lead you to getting the optimium performance right through your rev range.
In reality, most of us have stock needles or something very close to them. When we experiment with oversized rebores (raised CR's) aftermarket 4-1 exhausts, porting, cams, pods etc, the engines scavenging characteristics can drasticly change the demands on the mid range (needle circuit). To compensate for this, I have found that plug chop testing the pilot circuit up to 1/4 throttle confirms whether the transition air/fuel ratio is correct. Throttle response from idle and up to 2000 rpm, will tell if the pilot jet is too lean or too fat. Progressing from that, your plug readings at 1/4 throttle will influence you whether to raise or lower your needle position.
The test at 3/4 throttle is to confirm that the needle position is correct.
This is the point where the main jet takes over. In reality, the test is done slightly before 3/4 position.
All the plug chop tests should be done when the engine is under load, not cruising on level ground.
And carburettors with pump jets, well that changes the ball game again.

[KEITH KRAUSE]

Well, I must admit I wasn't even going to say a thing about what throttle positions should be used.
I've been a member here a long time and I know how easy it is for someone to think their toe's been stepped on. That was not my intention. 49er certainly makes an effort here to help and it's appreciated. He's helped me understand specific things better. Since it's his last reply here I can tell he thinks I stepped on him. He knows how serious I take carb advice and I'm sorry if it looked like I tried to improve on his method or whatever you'd call it. We both have our way and I guess the reader can do/decide what he likes.
My advice in this case, relates to the exact carb and generally the exact model the person asking for help has. He has VM carbs and a 78/79 1000.
The pilot circuit is best tested at minimal throttle. Beyond that, the slides cut-away briefly takes over before the jet needle takes over. On the VM's, 1/4 throttle is well into cut-away range and there's no sense testing the pilot circuit when you're in the cut-aways range. The DJ jet needles do allow the jet needle to have effect sooner than the stock needles. At 1/3, or some would even suggest 1/2 throttle, you're positively on the jet needle. At approx' 3/4, there's too much overlap between the needle and main to positively test for the main there, so full throttle is no doubt for the main. I've always advised marking your throttle if you have any doubt at where the throttle is. I suppose it could add a bit to the danger of high speed testing but I thought testers capable of it. I can't get into how various models use different throttles and how it effects the amount you are truly opening the carbs. Too technical. I just open the throttle to the needed amount. I've never personally needed to mark a throttle, but some may benefit from it. Their choice.
Also, testing under load is correct when reading plugs, but when testing the pilot circuit, minimal throttle should be used and is adequate. Such small throttle openings and being under a load don't really go hand in hand. I mean, you must open the throttle beyond the correct range to really apply a significant load. Moderate acceleration isn't what's needed here. A steady speed around town, just general/easy riding around, will give you a fair idea of what the pilot circuit is doing. On the other two circuits in question, 1/3 and full throttle are certainly load related enough.
My opinions on best throttle positions are based on a true separation of the circuits. Minimal, 1/3 and full meet that need.
Anyway, sorry to get on anyones nerves. Smiley...see?

[Einar J Skog]

Hi

About cam clearence..


I have set the valve clearence to 0.1 mm (0.004 inch)