Carb Interchange No Start

Hello have a unique situation here I have 2 bikes here both are 81 xj 650 Maxims. one is a Yics and the other is not. I am planning on swapping the Yics into the other frame as the bike is hardtailed and customized and the non yics motor is beaten up after being stolen and returned with all the exhaust threads broken and no way to mount the exhaust.

My plan is to swap the wiring harness and motor onto the new frame. I purchased the Yics bike for 300 dollars with no title and the bike started right up with starting fluid shot into the carb. I have had my other xj for quite some time and have become somewhat knowledgeable on the church of clean, information overload and what not. So i imagined that the bike would not start normally because of the carbs so I bought it.

When I got the bike home i switched the carbs out with my set. I was able to get them started for about 3 minutes. I have since not been able to get it to start. it cranks but will not fire. This carb from my non yics bike has been thoroughly cleaned according to the church with throttle shaft seals replaced in 2018, as well as ultrasonically cleaned at the same time. I have never had any issues with the carb starting the other bike. I have tried using a fully charged battery and even a car battery.

The carbs are jetted for pods and a 4 into 1 exhaust, i do not have the air-box on the carbs for the yics bike but I do have the factory exhaust on. I know thats not the right setup but shouldn't it start with the bigger jets? I have removed the float bowls and removed the jets and made sure they were clear. no floats are sticking and the pilot fuel jets in the float bowl are clear I checked with jet spray and compressed air. The plugs are firing as I checked with them out of the cylinder head. I removed the plug wires and it seems that the plugs look fairly dry when cranking without the wires attached which to me suggests they are not getting fuel. I checked the fuel air mixture screws and they are all at about 4 turns out. I am at a loss of what to check next. if I had just one internal passageway blocked inside a carb wouldn't the bike still fire on other cylinders?

I am about ready to order new throttle shaft seals from Chacal for the carb rack that came with the Yics engine but really wanted to see the bike run properly in the original frame before I took the time to remove it and put it in my hardtail frame. I have that rack tore all the way down now and cant put it back on the bike to isolate any issues with the other carb until I rebuild it. I have not been able to test the transmission and wanted to see it running properly before I spend more time on it.

Any help on how to troubleshoot would be appreciated.

 

Can you verify that there is fuel in the float bowls?

With the re-jetted-non-yics carbs, will it still run briefly with start fluid sprayed in?

 

Following, sounds like what's happening to my bike

 

I pulled the carbs off and there is fuel in the float bowls. today i switched all the jets in the bottom side to brand new jets. i had some new 43 and 112 jets and I tried the same tests. tried starting it normally nothing, took fuel air mixture from 2 turns out to 4 1/2 turns out, checked plugs after cranking it without plug wires. still dry. i am getting new throttle shaft seals and gonna start from scratch with the other carbs. Ill figure it out, I was just hoping to get the thing running with the old carbs before I switched the motor from 1 bike to the other

 

Mezzmo's video 2.26 minutes in. Did you clean the cold start jets?

 

I did clean them I am concerned that maybe there is blockage in the brass tube itself though.

 

Are you talking about the brass tube the drops down into the well in the bowl for the enrichment circuit? You should be able to blast carb cleaner through that and have it squirt out by the throttle plate. And obviously the little jet from the main bowl into the enrichment circuit well needs to be able to shoot cleaner through it. Just watch your eyes.

 

Yep I have got my eyes from cleaning that tube. waiting on payday to get throttle shaft seals from Chacal. Man them little pieces of rubber are expensive...

 

That we can even get them is a miracle, seeing as Hitachi shut down their carburetor division over 20 years ago. Thankfully there is enough demand that other companies still make most of the parts.

 

So did the compression test tonight terrible results Cyl 1-75psi #2 90psi #3 85psi #4 90psi. Did I buy a dead motor?

Here is how I did the compression test: Took out all spark plugs hooked up gauge to each cylinder and cranked starter till gauge stopped rising. Full battery. throttle was cracked all the way each test.

If I did get a bum motor is it possible to take ,the top end off the yics motor, and I did a VIN check and its an 82 not an 81., and put the top end on my 81 non Yics? Can I frankenstein the two?

 

I guess why did the thing fire up with starting fluid if the compression is so bad

 

compression gets significantly higher with oil in cylinder over 125 I did not test all cylinders with oil

 

Stuck piston rings cause low compression. Use a 50/50 mix of acetone and automatic transmission fluid, a small amount in each cylinder for a few days to free them up.

Check your valve clearances too.

 

I think I outlined the problem earlier. My original 81 motor has broken exhaust threads.

Yeah I guess since the thing started with Stating fluid, that I thought the compression was good. If the piston rings were stuck wouldn't they have been freed when it started with starting fluid?

 

Also looking at the bottom half of the motor on the 81 non YICS and 82 YICS there is a difference between the lower ends, heat shrink fins are too large to allow the YICS cylinder head to mate up because of the YICS tube. Bummer... I did find a non YICS cylinder head on Ebay. I know that the the non YICS motor is good. Any issues I should be concerned about if I switch the cylinder heads on two non YICS motors? Change gaskets, maybe change rings since its opened up, obviously valve clearances. Isn't there a chain guide that goes bad in these motors, can I get to that with just the cylinder head off?

 

I think the motor would need to run a bit longer to free them.

 

It will. Rings don't free up instantly, especially if they haven't been soaked in a solvent.

 

I finished the rebuild of the carb that came on the YICS bike. I ended up putting the throttle shaft seals that were on the nonYICS bikes carb in the Yics with a full rebuild kit. I put the carb on the bike last night added gas and VOILA it started right up. I have not synced the carbs with gauges yet and I know the shims will need done but the motor sounds pretty strong with just gas and it runs pretty smoothly with just a bench sync. I didnt ride it just ran it in my garage as it was getting late. gonna button it all back together tonight and sync carbs. I lost part of my colortune so I wont get that done for fuel air but right now they are set at 2.5 turns out.

So thats promising. I am still gonna check the compression on the cylinders after it has ran with gas and see if those numbers came up. If It did I am just gonna remove the motor and stick it in the hardtail frame.

I am uploading some pics of the hardtail from before. Also a pic of an cafe style xv750 I did for a friend. This bike is about to live again my friends.