84 GPz750 bogs and dies once hot

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84 GPz750 bogs and dies once hot

14 Aug 2025 08:46 - 14 Aug 2025 08:48
#915958
 Just finished up fixing some electrical issues early this week, buttoned up everything and got my bike out of the garage to let it fully warm up and take it for its maiden voyage in the neighborhood for the first ride in 8 years.

Let it idle 5-10 minutes in the driveway and went to give it a few revs and it stumbled but seemed to be idling okay. Thought fuel in the tank was too low so ran and got a few gallons of fuel and put about a gallon in the tank. Bike was off about 20 minutes. Started it up and gave it a few revs after 2-3 minutes and it was snappy again.

Pulled out of the driveway and couldn’t wipe the smile off my face it was running so well. Made it a few blocks and noticed it was bogging again with throttle. Got to the stop sign and it shut down. Started it up but it still wanted to die with throttle. Pulled the choke out and the revs shot up and I made it most of the way home in 1st just feathering the clutch. 

Quick search on here says valve clearance, poor A/F mixture, or an issue with spark. Any symptoms I’m describing point to a starting place?

Also, carbs have been off and cleaned and checked over twice in the last year. Most recently I put in 117.5 mains chasing a lean condition (have 112.5 as well as 110 stock). Have stock air box and a delkevic 4 into 1 pipe.

Attached pics of each plug. Running NGK BR9ES’s.


     
Last edit: 14 Aug 2025 08:48 by 84_GPz750.

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  • Wookie58
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Re: 84 GPz750 bogs and dies once hot

14 Aug 2025 09:41
#915960
How old are the carb to head rubbers ?

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Re: 84 GPz750 bogs and dies once hot

14 Aug 2025 09:45
#915961
Purchased new in May of last year. 

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Re: 84 GPz750 bogs and dies once hot

14 Aug 2025 11:48
#915965
Purchased new in May of last year. 
Have you tried opening the gas cap when it loses power (blocked vent) ?
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Re: 84 GPz750 bogs and dies once hot

14 Aug 2025 12:42 - 14 Aug 2025 12:43
#915969
Check the basics.

As Wookie says, make sure the gas is flowing correctly.  You may want to open a couple of float bowl drains, to make sure the bowls are filled.

Check your charging system.  Put a DC volt meter across the battery, and rev up to 5000 rpm.  You should have between 14 - 14.5 volts DC.

And needless to say, check valve clearances.  This is just normal maintenance, often neglected.

You may want to check the link in my signature about "things to know".  Something overlooked may ring a bell.
Last edit: 14 Aug 2025 12:43 by Nessism.
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Re: 84 GPz750 bogs and dies once hot

14 Aug 2025 13:23
#915971
Purchased new in May of last year. 
Have you tried opening the gas cap when it loses power (blocked vent) ?
I did not. Although it was obviously opened when I filled the tank with a gallon of fuel so you might be onto something. I had disassembled the gas cap to paint it and thought I got it back together correctly. This morning was the first time it had ran with the gas tank hooked up since doing that.

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Re: 84 GPz750 bogs and dies once hot

14 Aug 2025 14:11 - 14 Aug 2025 14:17
#915975
First off your running the wrong heat range plug, run a B8 heat range plug (NGK or ND equivalent) Try to get a non resistor (no "R" pn) plug.

Stock jetting for that engine/carb combo assuming stock CV carbs is #37.5 pilot and #110 main jet, a #117.5 main is way too rich.

With a stock air box and that pipe you will be OK dropping to a #110 main.

If it has not run for a while make sure you blow spray carb cleaner thru all the pilot circuit orifices. There are several pics available on this site.

Do you know that the float level is correct? Are the choke plungers sealing?
 
1978 KZ650B2 w 1197cc Z1 engine
1977 KZ650B1 w 750cc Spectre engine
1979 KZ650C3 w 831cc Hot Rod engine
1978 KZ650C2 w 762cc DFI project
1977 KZ650C1 stock restoration project
1978 KZ650B2 modified project
1978 KZ650B2 Injected Drag 831cc
1980 Z1 Custom Frame Drag 1327cc
1981 Z50R Honda tow bike
Last edit: 14 Aug 2025 14:17 by Injected. Reason: info
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Re: 84 GPz750 bogs and dies once hot

15 Aug 2025 04:35
#916009
Thanks for the info @Injected. You say the wrong heat range plugs. The manual I have shows BR9ES for standard riding and BR8ES for low speed riding? I picked up a set of NGK B8HS non resistor type that I’ll try. 

I’ll toss in the smaller main jets. I’ve been starting it up a few times a week since spring but lately had a battery overcharging issue so it would only be running a few seconds while I was troubleshooting. Early this spring I pulled the carbs off and double checked my float heights but two of my drain screws are stripped so I cannot confirm the clear tube method level of correct. 

I will have to do some reading on the choke plungers as I am not sure how to verify that.

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Re: 84 GPz750 bogs and dies once hot

15 Aug 2025 04:44
#916012
Check the basics.

As Wookie says, make sure the gas is flowing correctly.  You may want to open a couple of float bowl drains, to make sure the bowls are filled.

Check your charging system.  Put a DC volt meter across the battery, and rev up to 5000 rpm.  You should have between 14 - 14.5 volts DC.

And needless to say, check valve clearances.  This is just normal maintenance, often neglected.

You may want to check the link in my signature about "things to know".  Something overlooked may ring a bell.
This morning I cracked open two carb drains and had a few oz of fuel each. Did this before popping the fuel cap open for the first time since having the issues yesterday. 

I’ll test for voltage once the neighborhood is awake this morning. Don’t want to quickly become known as the guy with the loud old motorcycle 

Checking valve clearances is something I’ve never done but I have that portion of the manual sitting on the bench. What is a good source for shims should they need adjusted?

I’ve attached a video I took yesterday when I initially gave it a few revs after it had been idling for 8-10 minutes. I had given it 3 or 4 quick revs and got the results attached. It wasn’t one gradual rev that was sputtering. 

    IMG_2670.mov

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Re: 84 GPz750 bogs and dies once hot

15 Aug 2025 04:55
#916014
Yes, the factory manual calls for NGK BR9ES or ND W27 ESR-U, so the ones you have are correct. One way to check the choke plunger is to pull one out and look at the seal in the tip. If it has a deep indentation or is really hard, they need to be replaced. 
1982 KZ1000 LTD parts donor
1981 KZ1000 LTD awaiting resurrection
2000 ZRX1100 not ridden enough

www.kzrider.com/11-projects/620336-anoth...uild-thread?start=24

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Re: 84 GPz750 bogs and dies once hot

15 Aug 2025 05:57
#916018
Its a stock 750cc air cooled engine, every one I have ever seen runs B8 plugs.

B9's would be fine on a race bike that runs much hotter, I run B8's on my 90HP 831cc street bike. I run B9's on my 831cc alcohol injected drag bike and my 1327cc gas drag bike.
1978 KZ650B2 w 1197cc Z1 engine
1977 KZ650B1 w 750cc Spectre engine
1979 KZ650C3 w 831cc Hot Rod engine
1978 KZ650C2 w 762cc DFI project
1977 KZ650C1 stock restoration project
1978 KZ650B2 modified project
1978 KZ650B2 Injected Drag 831cc
1980 Z1 Custom Frame Drag 1327cc
1981 Z50R Honda tow bike
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Re: 84 GPz750 bogs and dies once hot

15 Aug 2025 07:46 - 15 Aug 2025 07:47
#916019
So this morning I’ve checked a few things. Battery voltage at idle was reading 14.4V and when revved to 5k would lower slightly to 14-13.9V. 

Pulled the tank and side covers off because it ran fine while just using an auxiliary tank, it ran the same (poorly) when back on the aux tank. Seems to stumble and lug down off idle to 1800-2000 rpm then opens right up. 

I checked the vacuum lines to the carbs to make sure I didn’t mess one up or knock it off when reinstalling everything. When that all checked out and it still wasn’t running well I pulled the carb rack. Installed the 112.5 mains and tossed them back on. It arguably is running worse now. Rather than clearing up around 2K, I can hold it at 3k and the revs will be bouncing all around and then out of no where clean up and hold the rpm. 

When I pulled the carbs off, there was plenty of fuel in the bowls and I could see some residual on the intake side of the motor. 
Last edit: 15 Aug 2025 07:47 by 84_GPz750.

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