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Jetting changes due to elevation? and top end rebuild questions.

Started by e30 gangsta, October 14, 2021, 11:45:59 AM

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e30 gangsta

Hi fellow 2 stroke friends. I am moving from Savannah, Ga to Denver CO and I am a little worried about how the bike is going to run at that elevation. I would assume that the oxygen density is lower in Denver than in Savannah due to the elevation. Therefore the bike would be a little fat/rich for its new environment. The doubt that I have is will I have to change the jetting? We already run the carbs on the leanest clip setting on the tm34's. So all I can really change is the pilot and the main. Curious how you guys go about adjusting carbs when running them at a higher elevation.

Any tips or tricks would be greatly appreciated.

Also since I am moving to the other side of the country, Chuck will no longer be able to assist me on maintaining the bike. Besides logging miles, and top end noise is there any other way to know/measure when new pistons are required? Might be a dumb question, but I was curious if there was a was a way to measure wear or any tips/tricks.
Because If I have to visually inspect the pistons, then ill just send the jugs off to be bored, and order up a new set of pistons. Not that the bike needs a top end rebuild currently but I want to prepare myself for when that time comes.

Any solid engine builders near CO? Only 2 that I know of that are still doing work is Garret, and SCR? I'm open to other recommendations.

Thank you.

pdxjim

Properly maintained, you could daily ride an RD400 for 10 years without "needing" a rebuild.
Wasting time on 2T forums since the dawn of the internet. '89 TDR250, '13 300xcw, '19 690smcr, '56 Porsche 356A

m in sc

IF its an issue...run a jet tube leaner and start w the needle in middle jet from there. that's a significant change in altitude.

SoCal250

With +5000 ft altitude change the carbs will definitely run richer.
75 Yamaha RD125B   75 Yamaha RD125B (project)
75 Yamaha RD250B   75 Yamaha RD200B (project)
73 Yamaha RD350     77 Yamaha RD400D   79 Yamaha RD400F  
91 Yamaha TZR250R  89 Yamaha FZR400   05 Yamaha FZ6   
05 Yamaha XT225TC  82 Honda MB5  02 Aprilia RS250 Cup (sold)

e30 gangsta

Quote from: pdxjim on October 14, 2021, 01:18:39 PM
Properly maintained, you could daily ride an RD400 for 10 years without "needing" a rebuild.

Not at this level of tune. Every 7500 miles I have to put in new pistons. It makes double the whp from what it made stock.

SoCal250

If you check out the Mikuni Pocket Calculator it has elevation correction.  For example, a change of +5000 ft means the main jet (4/042 type) should be downsized from a 220 to about a 200 (actual calculated value of 197.5). Temp and humidity changes will also vary the output.
75 Yamaha RD125B   75 Yamaha RD125B (project)
75 Yamaha RD250B   75 Yamaha RD200B (project)
73 Yamaha RD350     77 Yamaha RD400D   79 Yamaha RD400F  
91 Yamaha TZR250R  89 Yamaha FZR400   05 Yamaha FZ6   
05 Yamaha XT225TC  82 Honda MB5  02 Aprilia RS250 Cup (sold)

e30 gangsta

Quote from: SoCal250 on October 15, 2021, 12:22:23 PM
If you check out the Mikuni Pocket Calculator it has elevation correction.  For example, a change of +5000 ft means the main jet (4/042 type) should be downsized from a 220 to about a 200 (actual calculated value of 197.5). Temp and humidity changes will also vary the output.

That is a great resource. I will look at the carbs in a couple of days and see what jets are in there and go from there.

Yamaha 179

Quote from: e30 gangsta on October 14, 2021, 11:45:59 AM
Hi fellow 2 stroke friends. I am moving from Savannah, Ga to Denver CO and I am a little worried about how the bike is going to run at that elevation. I would assume that the oxygen density is lower in Denver than in Savannah due to the elevation. Therefore the bike would be a little fat/rich for its new environment. The doubt that I have is will I have to change the jetting? We already run the carbs on the leanest clip setting on the tm34's. So all I can really change is the pilot and the main. Curious how you guys go about adjusting carbs when running them at a higher elevation.

Any tips or tricks would be greatly appreciated.

Also since I am moving to the other side of the country, Chuck will no longer be able to assist me on maintaining the bike. Besides logging miles, and top end noise is there any other way to know/measure when new pistons are required? Might be a dumb question, but I was curious if there was a was a way to measure wear or any tips/tricks.
Because If I have to visually inspect the pistons, then ill just send the jugs off to be bored, and order up a new set of pistons. Not that the bike needs a top end rebuild currently but I want to prepare myself for when that time comes.

Any solid engine builders near CO? Only 2 that I know of that are still doing work is Garret, and SCR? I'm open to other recommendations.

Thank you.
I don't have any experience with TM series carbs but I have plenty of experience in jetting VM carbs, and plenty of it in your neighborhood, Roebling Road Raceway in Pooler, GA and other race tracks east of the Mississippi.  SolCals suggestion of using the Mikuni pocket calculator is a good one and you can buy one from Sudco, International for a few bucks.  Set your Savannah (sea level) jetting then enter your Denver level jetting, fudge a size or two (for safety) and try it out.  As for piston replacement time; you should determine that by careful piston/bore comparison.  Measure the piston with a micrometer.  Measure the cylinder bore with a dial bore gauge.  The difference inn size determines when the piston(s) should be replaced.  Cast piston tolerance is tighter than forged piston tolerance.  The equipment needed to do this work is expensive and might be beyond your expertise.  Perhaps you should send the work out, but beware; many local shops can no longer do this work with any degree of accuracy because modern engines (four strokes) don't require much of it. Compare the two and the difference determines if you should replace the pistons.  It also tells you if the bore is straight or out of round/tapered.