Bogging down at 7.5k RPM

[thumperjsa]

It feels a little like fuel starvation. I put the petcocks on Prime and it gets a little better but not significantly. I have not tried opening up the fuel cap and trying that yet.

I looked around the forum. Anything else in particular I can check into before I start driving my self crazy or spending money I don't have?

I suspect the rings are a little weak because of the blow by through the crankase tube and also I did a test for compression and I had a bit above 100 on 2 cylinders and about 120 -125 on the other two. I put some Marvel oil in the cylinder and got an increase. One cylinder went from 100 to 150 so I know the rings are letting some go by. I am going to wait until the bike starts running more poorly before I do an engine tear down.

I also noticed since I put on the new 4 -1 MAC exhaust I don't have the pickup I had. The old exhaust header had a pin hole and a bad repair job under one of the pipes and the kerker pipe did not do much in the way of quieting down the bike if you know what I mean.

The bike runs nice and quieter since the new exhaust but it is not trying to pull the arms out of my sockets at 2500 RPM like it once did.

By the way, I am waiting on some K&N cone filters to replace the airbox. I know I may have to change the jet. I have to first determine which one it is. The PO did by the airbox off of Ebay and I don't know if it was because it had cone filters on it or not previously. I don't talk to him because he was an a** and did not tell me about the problems before I bought it off of him through Ebay.

Oh well. you live and you learn. This is my project I took on and I will see it through and enjoy riding it as long as my wallet and my wife holdout.


-Jonathan

[chevy45412001]

air box removeal

be advised,i took off my stock air box and did not like the results.I lost a ton of mid range power.It took a long time and many jet changes to get it back to almost normal.(mines an XJ) stock 112 jets,now running 137.5's.If your lost on top end which 7.5 thous rpm is,i'd kick up a few jets sizes. But actualy i'm looking to put back on my stock box,it just plain ran MUCH BETTER. But hay thats me.

[thumperjsa]

I ran yesterday on prime and that made no real difference. So I tried opening up the gas cap a little and run it that way. I thought it was really making a difference because I was getting towards 7500 and no bogging down. Before I could see if it was the problem I had to back off.

Next time I try it, I will wait until the fuel level is not near full in my tank. Glad I don't smoke if you know what I mean.

By the way, if it is the vent in the cover that is clogged, how do I unclog it? I saw someone mention drilling a bigger hole but that seems odd.

[ratbyk]

Plugged Vent

To clear the vent hole on mine all I did was take an ice pick type tool, ream it out a bit and then hit it with compressed air to blow out any crud etc. No problems with it since I did this. I have heard of people taking the cap apart to do but I was successful with my short cut fix.
Ken/Sooke

[TopCatGr58]

Hey there Thumper,

Okay, you put 4-1 pipes on, this will increase your topend performance, but you DO loose some bottom end torque due to reduced back pressure. You should probably increase your jets anyways, 2 sizes for the pipes, and when you get the Indy filters, another size more! I've seen your posts before, but I can't remember if you stated that you've cleaned your carbs or not!?

You could also try setting your floats a mm lower(upside down). Have you checked your timing and vacuum and centrifugal advance mechanism? The lower compressions should not keep you from reaching redline. You may also want to put in a thicker grade of oil like 20/50 to help the rings seal better!

Chevy mentioned his problems he had by removing his airbox, but he has an XJ, and they have that funky YICS system, makes tuning much harder as Mike Hart can attest to!!
Good Luck. T.C.

[MartyA]

Might be as simple as the tank vent. TC has a point about checking the timing. Check the vacuum advance plate to be sure it moves freely, and the vacuum is pulling the plate. Also check the centifugal advance behind the timing plate, be sure it is not gummed up.

[thumperjsa]

Once I get some of the fues out, I will try opening up the fuel filler and running again to see if fuel venting is an issue. I will also check the vacuum advance.

I know that I can't currently get my bike above 8k RPM because it bogs down at that point.

[John]

Originally posted by thumperjsa
Once I get some of the fues out, I will try opening up the fuel filler and running again to see if fuel venting is an issue. I will also check the vacuum advance.

I know that I can't currently get my bike above 8k RPM because it bogs down at that point.

How freakin' fast do you want to go?

[thumperjsa]

Maybe what I should have said was 8k in lower gears. 8k in 2nd is only about 70 MPH according to the chart although I don't think I am going that fast in 2nd gear. since readline is above 8k, should not I be able to get there? I will have to go back and check the math versus reality.

[ratbyk]

Bogging @ 8000

Thumperjsa, before you flirt with disaster at RPM's like that you had better check your tach and make sure it is accurate. It could quite possibly be reading a 1000 RPM low meaning you are bumping up against 9000 RPM. Another possibility of leveling off at 8000 Rpm is valve float. Remember you are operating with 25 year old valve springs that just might be giving up controlling the valve train at extreme revs. If that is your problem and you keep letting those valves just rattle around out of control one is going to tip a piston then you can kiss goodbye to your motor. When things start to come apart at 8000 not much is usually left to salvage. A head will break of a valve, the piston will jam it into the head destroying it, as the piston breaks up it allows the rod to swing free and it starts cutting the cylinder and crankcase in half. It happens so fast and the motor jams up so quickly you cannot believe the amount of damage that took place in less than a heart beat.
Shift a bit earlier, stop trying to go 130 MPH and both you and your scooter will last a lot lonnnngggger.
Ken/Sooke
78E Ratbyk
82 FT500 "lilRat"

[John]

8k in 3rd is about 85 mph. That's where I missed a 3-4 shift and put an 'owie' on my old engine. Bouced the tach needle of the low fuel light on my Special, about 12k rpm I would guess.

[MartyA]

"...kiss goodbye to your motor."

Yeah, you should see the BIG hole it can make in the bottom. Got one of them. I didn't do it, came in a pile of parts I bought, has a chain drive conversion so it must have been in a little racer.

Heck, shift at 7K and still outrun 90% of what's out there, cage or bike.

My bike has never seen 7500 since I owned it. It has seen 7000 twice - the first time was when I bought it with 16K on the clock, the second time was a few weeks ago right after I adjusted my slipping clutch and wanted to test it.

I expect mine to last another 26 years. But thats me.

[thumperjsa]

More good news

I took a further look into things. Decided to go with external fuel filters and put them in. Before I started putting the filter, I noticed that the first cylinder was not getting as hot as the other 3. Then I pulled the plug and I was getting spark but there was no way I could tell if it was a hot enough spark. I could feel compression pushing air out of the chamber and I did previously do a compression test on it. It was not great but wasn't horrible either.

Then I put the plug in and restarted the engine. Still not gettting as hot as the other exhaust manifolds. So I put the fuel filters on and restarted the engine. I happened to have my head close by to the cylinders and I hear this ever so slight puffing/whistling sound. So I decided to get out some cold start fluid and guess what came next. The engine starts revving up when I spray on the number 1 cylinder intake boot. Now I am getting pissed. I paid the bike shop to rebuild the carbs and reseal the boots and it lasted just over a month. I sprayed all of the other boots and this was the only one apparently leaking.


So next I guess I need to buy new intake boots and replace them. For now until I get the new boots, I put some Threebond on it and I will pick up some 3m Liquiod gasket as some others have recommended. Threebond dries grey and I don't like the look of it. No wonder why it felt like after the carbs got done that the bike at 2500 RPM wanted to pull the arms out of my sockets and now she don't want too. I guess running on 3 cylinders instead of 4 will slow it down not to mention bog down an engine at high RPM.

By the way, I have not been able to get over 7500 RPM (highest I believe lately is 7K) and have no intentions of going 130 MPH on this bike, so you don't have to be concerned about that. For right now, no long trips on it until I close this chapter. Trust me when I say, I want to make this my long term project bike and I would really upset myself if I left parts of her all over the road.

One more thing, I see boots on Ebay for about $85 and Parts and more for $88. Those are 2 of the sources I have seen so far at under $40 a piece. Parts and more I also believe offers the free ship where Ebay does not. Anyone have a suggestion of any other sources?

[thumperjsa]

Other than the question of a source for intake manifolds, I have another question for my 80 G.

Below is the stock size for jets:
Main Jet Pilot Start Main Air Pilot Air Throttle Valve

115(1&4) 42.5 25 140 185 135
120(2&3)

Having 4-1 exhaust (up 2 sizes) and going to individual K&N filters (up 3 size) and then subtract 1, if I have my math correct and according to Jetting recommendation page, I should change the mains to 4 sizes larger and 1 pilot size (each size being 2.5 increase).

So I should go to:
Main Pilot
125 (1&4) 45
130 (2&3)

Is this correct and all of the other jets are left as is?

Is there a good source with reasonable prices and good delivery times to get both of these including carb gaskets?

[John]

Motorcyclecarbs.com he specializes in old japanese carbs. He usually has OEM stuff. There is a phone number on his homepage. Give him a call. I used him for my needle and seats, and I talked with him for a while. Good guy, knows his stuff. He has buddies in an XS750-850 triples club.