no vacuum @ idle?

[GARTHXS]

I am trying to get my 79f running and me and scott deduced that one of my coils had bad caps , but still not idleing , we tried to sync it but it is spitting thru the carbs and also no vac at idle speed ?? Any ideas??

[trbig]

When I was running the early cams from a '78 in my later motor, I had almost no vacuum. I'm not saying this for you to supect the cams, but apparently the timing has a huge part in this. When I put in the late model cams right after that, my vacuum rose a LOT.

So.. maybe with spitting and this vacuum problem.. you might start with your timing and make sure everything is on the marks and lines up. Including taking the valve cover off and checking the cams against your crank.

Just a suggestion. I'd start under just the timing cover first though. Something may be hanging your timing up in the advanced. I know those bearings that let the timing assembly move go bad sometimes.

Tod

[jmnjrpa]

Had the hose pull loose at the vacuum advance. Gave me fits till I found it. Simple to check.

[GARTHXS]

timing

Yeah that was my first thought too , but the timing checked out so far - I guess I am going to peel off the valve cover, scott thought I might have a hot cam as well , I will find out when I dig a little deeper and check valve clearances , this bike is pretty stock now but the PO was kind of a hot rodder. How can I check the cam itself?

[cycleguy]

i've had acouple xjs with the same problem-adjust the idle mixture screws one and a half turns out from lightly seated as a base. did bring the vac up & was able to sync carbs and fine tune idle mix. hope this helps.

[GARTHXS]

1/4

This set of carbs is off a 80g. With stock jetting. And pilots turned out 1/4 turn. I am going to tune up my 79 carbs and see if the vacuum changes as well I don't really want to mess witht valves this bike is for sale and I really don't want to spend much more money on it

[dbeardslee]

How can I check the cam itself?

There will be a number printed on the cam shaft. IIRC the '79's have 2H7 printed on them. Tod probably knows for sure.

[cywelchjr]

This set of carbs is off a 80g. With stock jetting. And pilots turned out 1/4 turn. I am going to tune up my 79 carbs and see if the vacuum changes as well I don't really want to mess witht valves this bike is for sale and I really don't want to spend much more money on it

1/4 turn out is probably too lean and this will cause you to need to have the idle speed set high and will cause very low vacuum. Adjust them out one at a time, idle speed will increase, and get them all out to highest idle (I then turn them in just a bit from there) and set the idle speed back down (usually while you do this you will need to adjust idle speed several times) then you should have good vacuum and be able to sync.

[GARTHXS]

oops!

What i meant to say was 1-1/4 turns out

[trbig]

Yeah, the 78/79 cams will have 2H7 stamped on them and the 80/81s and the XJs have 3H5.

Have you done a compression check?


Tod

[GARTHXS]

negative

not yet maybe when I get back , this bike sat for at least 8 years without running , I did the usual MMV in the cylinder trick , and it turns freely and when it did run it didnt seem to burn oil

[motoman]

Garthxs, try running idle mix screws out to 2 1/2-3 turns initially.......should help as these are the later carbs.

[foreda]

Low Vacuum

I had a similar problem with my 79F when I bought it. I ended up having to completly rebuild the carbs including shaft seals. I had another set I installed to comfirm my diagnosis and the vacuum was fine. My next thought would be like everyone else... Valves!

[DGXSER]

We had another members bike in Dbeardslee's mancave that had those same symptoms. A new to him "good" engine. Wound up being a bent intake valve.

[Athedra]

Read through the thread briefly. I usually sync my carbs on 2k rpm. Then adjust my air fuel mixture, and then resync on idle.

Do you have enough vacuum to sync on 2k?