Author Topic: 400/4 is popping & banging with poor mpg where to start?  (Read 791 times)

Offline McCabe-Thiele (Ted)

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400/4 is popping & banging with poor mpg where to start?
« on: June 16, 2022, 09:29:15 PM »
This evening I fired up the 400 from cold I let it run at between 2- 2500rpm for some 20 seconds then checked the front pipes.

Number 1 & 3 cylinder are virtually cold - number 2 & 4 nice & hot.

So where do I start to diagnose what the problem(s) are please?

An Idiots guide if you will of the best logical steps I need to take in sequence please?
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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #1 on: June 16, 2022, 09:40:41 PM »
Tap the float bowls Ted in case the floats are stuck shut.
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Offline K2-K6

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #2 on: June 16, 2022, 09:55:22 PM »
Also get the plugs out and post decent pictures of  them to help identify problem.

Are the 1+3 just wetted with little combustion?

Offline McCabe-Thiele (Ted)

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #3 on: June 17, 2022, 06:43:49 AM »
Yes I clearly need to remove the plugs & see what's going on with cylinders 1 & 3 thanks for the initial pointers.

Off to Wales for this weekend with a planned boat trip over the Pontcysyllite Aquaduct - bike will have to wait until Monday unless I can sneak the plugs out before we go.

I've only ever walked over the Aquaduct when I was a teenager in 1968 - it's just taken me a while to re-visit the marvelous piece of Telford engineering.
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Offline TrickyMicky

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #4 on: June 17, 2022, 08:25:57 AM »
Enjoy the aqueduct, Ted. It's quite an 'interesting' perspective if you admire the view from the West facing side, (not the towpath side!). Mike.

Offline bruxby-clive

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #5 on: June 17, 2022, 08:47:29 AM »
Hi Ted, where did you get your plugs from? I had an intermittent fault on my KH 250 which I finally traced down to a sparkplug breaking down, my fault as I bought them off eBay, as soon as I replaced the plug normal service was restored. Although I do appreciate that as it is with 2 cylinders it is unlikely but worth a check.
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Offline McCabe-Thiele (Ted)

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #6 on: June 17, 2022, 11:49:26 AM »
Enjoy the aqueduct, Ted. It's quite an 'interesting' perspective if you admire the view from the West facing side, (not the towpath side!). Mike.

I guess you mean the sheer drop looking over the side of the tank sections - yes looking forwards to it.

I did visit briefly once since 1968 with a fellow student only to find the hand rails were missing on the tow path side - it was a windy day so what started as a walk ended up with me crawling a short distance before common sense prevailed and I turned back.
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Offline McCabe-Thiele (Ted)

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #7 on: June 17, 2022, 11:51:32 AM »
Hi Ted, where did you get your plugs from? I had an intermittent fault on my KH 250 which I finally traced down to a sparkplug breaking down, my fault as I bought them off eBay, as soon as I replaced the plug normal service was restored. Although I do appreciate that as it is with 2 cylinders it is unlikely but worth a check.

The plugs were purchased from e-bay via UK seller that had good feedback - I have a new set from DS in reserve.
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Offline McCabe-Thiele (Ted)

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #8 on: June 17, 2022, 11:58:58 AM »
Mini update I have removed Plug 1 photo below it was clean but wet with fuel.

Plug 2 looked pretty normal - slightly on the rich side perhaps.

I'm confused as all 4 manifolds show heat discolouration as 1 & 3 have a cold front pipe find it hard to believe it would run on just 2 good cylinders.

Plug No 1 by Macabe Thiele, on Flickr

Number 2 below.
Plug No 2 by Macabe Thiele, on Flickr

Numbers 3 & 4 will have to wait until next week as I need to do some tasks round the house first.
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Offline K2-K6

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #9 on: June 17, 2022, 12:21:35 PM »
#1 definitely wet with not enough spark-age or too much fuel-age, question is which one?  #2 looks fine for combusting (slightly in shadow to see exact colour) but clearly dry and carbon as expected from continuously running cylinder.

Could be as Ken says, a sticking float valve that is giving it a bit of a soaking.

The #1 plug, just try it for sparks in another system to see shat it does (independent ignition scource) that will point you to what's happening and give you clear answer if plug will spark at least.

Enjoy the aqueduct trip Ted, often seen it on TV but not visited and looks pretty spectacular to cross it.
« Last Edit: June 17, 2022, 12:40:09 PM by K2-K6 »

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #10 on: June 17, 2022, 01:16:15 PM »
What plug caps are you using Ted as the screw on top nuts are still on yours and if your using NGK caps they don't tend to have them as they use the threads to grip.

A better picture of No1 plug would help to see if it's a cheap copy, look for smudging on the lettering, the top nut should also have a small indent, the crush washer isn't a good copy and tends to come off when pulled down. Also look for machining marks on the metal parts.

No1 doesn't look to have fired as it's wet through, which is worrying as you need to find out why it's like that.
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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #11 on: June 17, 2022, 01:34:16 PM »
After a little research Ted I'd say the plugs are copies, the electrode should have a curved surface and the copies are known to look very square looking. Yours look square to me.

They are known to fail very quickly, the NGK symbol is quite bright whereas the genuine can look faded. If you had the box you can tell as the box is bigger and has some wrong writing on, like TSE when it should have EAC. Amazon are known for having sold a lot of copy plugs. Maybe get them from a known source is better, like The Green Spark Plug company.
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Offline K2-K6

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #12 on: June 17, 2022, 02:20:11 PM »
Well spotted Ken, I hadn't looked at that end  :)

Definitely check the caps Ted as all the NGK I've used are without those ferule screw tops. They'll fire but sporadically as the spark needs to jump across from HT points to the ferule which will sit out of contact with it.

Likely if this is the case only two plugs will work as each coil being double ended has an HT polarity that maybe more effective in positive or negative orientation, and so two of them may tolerate better than the other two.

Offline McCabe-Thiele (Ted)

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #13 on: June 17, 2022, 03:22:13 PM »
What plug caps are you using Ted as the screw on top nuts are still on yours and if your using NGK caps they don't tend to have them as they use the threads to grip.

A better picture of No1 plug would help to see if it's a cheap copy, look for smudging on the lettering, the top nut should also have a small indent, the crush washer isn't a good copy and tends to come off when pulled down. Also look for machining marks on the metal parts.

No1 doesn't look to have fired as it's wet through, which is worrying as you need to find out why it's like that.

I'm no expert at spotting fake NGK plugs they cost me 10.99 for a set of four the packing looked genuine NGK, the washers are well fitted in place etc, I have a NGK spare set from DS I will try them.

I'm just using the plug connectors that came with the Dyna Kit  8mm silicone leads with the carbon string type centre.
Ideally I would have liked ready made leads but no local supply now. The local auto spares shop closed during Covid - it's now an Estate Agents, buying fittings locally seems a thing of the past.

My spark plug ends are just a straight metal connector with a moulded rubber type end came with my Dyna Kit - it needs the top nuts on the plug to fit securely.
I cut the leads to length crimping on the Dyna coil connectors - they are like the old BMC coil type fittings - I used a crimping tool.

Personally I prefer copper wire leads as you know there is a good connection - I'm minded to make a new coil end for No:1 lead as it's long enough.
I will fit a plug on the end of the lead to check for spark first.


« Last Edit: June 17, 2022, 03:24:12 PM by McCabe-Thiele (Ted) »
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Offline McCabe-Thiele (Ted)

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Re: 400/4 is popping & banging with poor mpg where to start?
« Reply #14 on: June 17, 2022, 03:39:01 PM »
Fake NGK plugs obviously a brand worth faking then.


I will try the DS set of NGK plugs with the leads as they are & see if I can get 4 warm front pipes for starters?

Brief Update: Number 3 & 4 plugs were exactly like Number1 & 2 virtually identical in colour etc.

I've fitted  the new NGK's from DS & run the bike for 10 seconds  still the same with No 1 & 3 manifolds being cold. I checked the spark on number 1 lead on an old plug there was massive
spark.

I've run out of Garage time now until next week when I return from Wales - do I check for compression next or am I missing something obvious before I take the carbs off?
I can access this site whilst I am in Wales as my phone does internet if there is a signal at the Hotel.



« Last Edit: June 17, 2022, 04:09:52 PM by McCabe-Thiele (Ted) »
Honda CB400 four super sport

 

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